The extreme profit oriented business culture of the US combined with the human nature of bandwagons make these sort of disgusting practices possible.
Corporations are justified, by default, in anything they can do to increase profit, and will do so until there's enough public backlash to negate the amount of profit that practice makes.
The public backlash is tied to the social momentum the idea has. Because profitability is the default idea to be promoted, you can't say something like "don't do this obviously profitable thing because it's bad for people" unless there's enough people around you who'll get on the bandwagon. If suddenly some influential person or a critical number of schmucks say the opposite, then everyone is defending the corporation's, not only the right, but the duty to be profitable.
It's an unpleasant way to live, really, but people are creatures of habit and won't easily go against the culture they grow up in.