I've sued Amazon (privately in arbitration) and I have friends who've sued them in court. Nothing wrong with suing to prevent infringement. It'll just get them to fix the problem, and you'll get nice money out of it if it's as clear cut as you say.

I very much doubt Amazon is rejecting DMCA notices, and suspect the story is more complicated than that. I've heard from many sellers that it's easy to file complaints, and I know many sellers who have gotten false complaints, which is also what you'd expect if it's super easy to file complaints.