Part of the reasoning for this, I think, is the Buddha's discouragement of monks and nuns from discussing certain unsuitable or 'bestial' topics: i.e., "conversation about kings, robbers, & ministers of state; armies, alarms, & battles; food & drink; clothing, furniture, garlands, & scents; relatives; vehicles; villages, towns, cities, the countryside; women & heroes; the gossip of the street & the well; tales of the dead; tales of diversity, the creation of the world & of the sea; talk of whether things exist or not" (AN 10.69).
If we live a worldly life, however, I think we, as 'householders,' have some responsibility to engage in worldly issues. While the Buddha clearly discouraged the monastic community from engaging in worldly activities such as politics, I think it's a mistake for lay-followers not to be. For one, politics affects almost every aspect of our lives, and being engaged in our communities and being a part of the political discussion, not to mention being active in broader social and political movements, is what makes our society and political systems function more effectively, and how progress, however slow it may sometimes be, is made.
To these these kinds of activities and decisions solely in the hands of others, some of whom are slaves to their defilements, isn't wise, in my opinion. And if we choose to live in the world, then I think we share some responsibility for shaping it; and it makes sense to have people motivated by things like non-greed, non-aversion, and non-delusion add their voices to the mix, not to mention helping do what they can to fix things like inequality and injustice as long as it's done with a spirit of compassion and harmlessness. The greatest danger of the practice of renunciation, in my opinion, is the tendency of practitioners to ignore the world around them while seeking their own happiness (which is one of the things that give non-Buddhists the mistaken impression that Buddhism is a selfish religion).
All too often in my experience, Buddhists fall back on teachings like 'all processes/conditioned things are inconstant, unsatisfactory, and not-self' (AN 3.134) while neglecting teachings such as "I am the owner of my actions, heir to my actions, born of my actions, related through my actions, and have my actions as my arbitrator. Whatever I do, for good or for evil, to that will I fall heir" (AN 5.57).
Moreover, just from a practical standpoint, not addressing many of the material conditions giving rise to and supporting society's suffering ultimately serves to help maintain their continued existence (when this is, that is), which can negatively affect our practice, as well as that of others. If the society one lives in isn't conducive to practicing Buddhism, for example, then it does matter what kind of society one lives, so we should naturally try to make it as conducive for ourselves and others as possible. As the Buddha said in Khp 5, "To reside in a suitable locality, to have done meritorious actions in the past and to set oneself in the right course — this is the greatest blessing." To help illustrate what I mean here, I'll give two example.
A general example is that a society that's not only consumerist, but also politically and economically geared more towards the idea that greed and self-interest is the highest good, will potentially be less supportive culturally of monastic communities that live entirely in an economy of gifts (e.g., in comparing Eastern cultures, in which alms-giving and gift exchanges characteristic of 'human economies' regulated by custom and reputation and based more on co-operation have historically been more prevalent, to Western culture, where market-based economies based more on competition have been the norm, I noticed that Eastern monastics often receive more lay support as opposed to Western monastics, who often have to produce goods like beer, chocolate, coffee, wine, etc. to sell in order to support themselves).
A more specific example is the ecological impacts of logging in Thailand. The Buddha praised the wilderness and the benefits of practicing in the forest. The Thai Forest tradition grew out of a movement among monastics to return to this way of practice. In the past few decades, however, much of Thailand's forests have disappeared, making this more difficult. Being involved in conservation efforts and trying to find better farming techniques and/or other ways of raising revenue is one way of trying to help preserve remaining forests in order to help keep this tradition alive.
The point is that, if the world is ruled by conditionality, doesn't it make sense that working towards contributing positive conditions for the benefit of ourselves and others is a skillful thing for householders to do? It'd be great if everyone were free from greed, hatred, and delusion, and everyone treated everyone else with kindness, compassion, and generosity—if the world was free from all forms of exploitation, privation, and gross inequalities. But the world isn't a perfect place, and we're not all saints; and one of the ways we can help alleviate some of the world's suffering is by trying to materially change it for the better. And from this point of view, it's not about making Buddhism political, but about applying the ideals of Buddhism in all that we do, which for me includes being socially and politically active.
Update: A friend of mine gave me a book for Christmas called No Beginning, No End, and I think the first three paragraphs of the foreword by Thich Nhat Hanh beautifully sum up what I'm trying to say here:
It has been said that the twenty-first century is going to be a century of spirituality. If it is not a century of spirituality, there will be very difficult times ahead for all of us and for the generations to come. If we are not able to stop and look more deeply at the suffering in ourselves, how will we be able to address the suffering in the world around us? In order for us to transform our own suffering, we must do something radical.
The first radical thing we can do to transform the suffering in ourselves is to practice stopping (shamatha). We stop in order to return to ourselves, to become calm. When we are calm, we have a better chance to see our suffering more clearly. The second radical act is to look deeply inside ourselves and see our suffering, be with our suffering, in order to understand and transform it. This is also true for the suffering in the world. We as entire nations need to stop and look deeply at the suffering in the world in order to see it more clearly without prejudice and understand how to transform it.
The practice of mindfulness in these troubled times is more important than ever. If we as individuals do not take the time to practice mindfulness, not only will it be difficult to transform the suffering in our own lives, but it will be difficult to transform the suffering in the world. It is vital to ourselves, our children, and the Earth that we have a practice that helps us to be mindful, that lets us come back to ourselves and dwell in the present moment in order to transform suffering in ourselves and others around us.
In other words, the practice and social engagement can go hand-in-hand and are entirely compatible.