A lot of models, even ones who never come to think of them as "promotional material", go through phases of freaking out when they find out their shows have been posted, to trying to get them taken down, and then to resignation to the fact that it's not worth getting worked up about them, since there's nothing they can do to keep them from spreading. Their energy is better spent on other aspects of their lives.
Some videos posted are from bulk cappers who've already been discussed here, who don't even watch them, and upload to them to sites that ignore DMCA notices. Some videos are posted by members who target models specifically, and if their videos are taken down on one site, they'll post them on a bunch more places so that at least some of them will survive. In the first case, MFC already knows which sites are a waste of time to send DMCA notices to. In the second case, sending DMCA notices will only make the problem, if there is a problem, worse.