The fact is, it isn't that important that Wikimedia Commons gets convinced that it's new copyright. A US-based company could ignore the CC-BY tag and happily reuse it violating the license, and you would have to assert your copyright (in order to enforce the license) in the court.
While that's a fair point, it's irrelevant. I realise there are limitations to how far the CC-by license will apply. However, I don't see how on earth that means I should have to give up my rights to it, or are all the photographers on this list going to give up their CC licenses because companies in countries where copyright is poorly enforced - China, say - could ignore the CC license and happily reuse it, violating the license, and they would have to assert their copyright (in order to enforce the license) in the court?
No, people are smart enough to recognise that making their things available means that their licenses might get violated, and that, in some cases, they may not be able to enforce the licenses. That doesn't mean that we say that Commons should only accept releases into the public domain.
-Adam Cuerden.