Keep in mind that the GPL is not an end in and of itself; it's a cute little legal hack to create a self-contained domain that simulates a world without copyright, with a one-way boundary to ensure that things stay inside that domain (there's a fairly straightforward game theory analysis of why this gives GPL-style licenses a competitive advantage over BSD-style licenses, but that's getting off topic).

So, in that context, it actually makes sense to simultaneously support strict enforcement of the GPL and oppose copyright.

Of course, most people don't think copyright is intrinsically immoral and thus won't find the above perspective persuasive, but I'm pretty sure that's roughly where people like RMS are coming from.