I think the idea is that a company with enough money (who hasn't done something that people would viscerally consider horrible) can influence enough people through advertising that the well-informed consumers "voting with their dollars" don't make a meaningful difference.

Is it really advertising that's at fault here? OP has a big issue with WalMart. I don't have a problem with WalMart at all. WalMart is an inevitability in a world where selection pressure rewards increased efficiency and lower costs. The most natural way to achieve that is through scale and centralization. But OP is clearly ideology driven and has a warped view of WalMart as some sort of super villian, that most normal people don't. He is confused why nobody follows him in his boycott and scapegoats advertising.