I suspect the actual reason you object to quitting is that it involves sacrifice. It's easy to sign a petition or hold up a billboard but it's hard to quit your job. The problem with this approach is that humans define virtue based on sacrifice. We are deeply suspicious of virtue that doesn't involve sacrifice and we should be because that kind of virtue is indistinguishable from self-aggrandizement.
If you really want to "question corporate power" then don't work for a coporation. Of course the reason people refuse to follow this advice isn't because "they want to change the corporations from the inside" -- it's because they want money and a lifestyle that comes from working for a coporation while also claiming the moral highground.