Pretty much all FLOSS licenses would be unenforceable. They only work at the moment because copyright prevents works from being distributed without permission, and the only permission you get for distributing such works is through the FLOSS license. If you don't adhere to the license terms (e.g. providing source, attributing authors, whatver...) you have no other way to distribute without breaching copyright.

So BSD licenses would be just as pointless if there were no copyright.