Perhaps John was not intentionally calling me out in his last post, but I have to register an argument in favor of Barrera's view, though I have not yet read Barrera's book.
In a competitive market, producers can be constrained to either participate in the market using the lowest-cost production methods, or not participate at all. This is why we like competition. This competition has a dark side, however, when low-cost production methods involve some sort of ethical compromise. In industries that are not competitive, producers might be said to bear some large amount of the moral culpability for legal but unethical production practices, but in competitive markets, it seems, we have to either maintain that no-one is culpable, or that the culpability is wider, encompassing the entire industry, and perhaps the government and consumers as well.
Ideally industry organizations or governments would regulate business practices so that competitive pressure is directed toward ethical production practices. This type of regulation, incidentally, is the topic of a paper abstract I have submitted to the ASE world congress. Absent this type of regulation, and especially when the state does not share all of the ethical convictions of a religious community, it seems that consumers, by paying attention only to prices and product quality, are morally complicit in, individually or collectively, the ethical harm. Even if one consumer cannot change the practices of an industry, they can support, with their dollars, alternative production methods.
John uses the contraceptive example, but my area of focus is, of course, the food industry. It would be odd to say that agriculture, and animal husbandry, is an industry that Christians should not participate in, and yet industrial animal agriculture, with all of it's ethical problems, is the only way to make a living for most farmers. Competition in the market for poultry and pork necessitates that farmers either sell at 50% above the market price, or adopt the standard industry practices. It is possible that similar arguments could be made about labor treatment in other agricultural markets as well. If we want to condemn these practices, and I do think we need to condemn some of them, then we will usually end up pointing fingers back at ourselves, either as consumers, for supporting firms that are competitive because they are unethical, or as voters, for not properly regulating these industries.
There is more to be said here. The biggest hole in this argument so far is that I have not discussed the possibility of industry self-regulation. I will have to deal with this in my paper, but it is a larger topic that would take me in a different direction.