The standing policy on mathoverflow.net is that it is not the community's job to enforce copyright. See http://tea.mathoverflow.net/discussion/313/
On Stack Overflow historically, moderators have removed links to copyrighted material whenever such have been made known. See flazx downloads: legal?
It is not clear what policy should be followed on this site.
Please consider, among other things:
Distinction between works that are not publicly available, but still under copyright, vs those that are still actively being sold.
Claims of violations coming from the copyright holder vs those flagged by users.
Differences in legal status between the various sites
This question grew out of the debate here over a moderator's action on a particular offer to provide copyrighted material in digital form. Read responses there for additional background and perspectives if you wish, but please confine discussion of the broader policy issue to this thread.