The fact that the GPL uses copyright is just an implementation detail. In a very real sense, the GPL is the opposite of normal copyright--it turns copyright law against itself. It goes further than not having any copyright (e.g. public domain/BSD/MIT): it not only ensures this particular work is free, it ensures all the works it can affect also remain free.
So the issue (from an idealist's standpoint) is not that the copyright of a GPL project is infringed but rather that additional limits are added on the project. The idea behind the GPL is to ensure that not only can everybody access the program, but they can legally and practically modify it. The people infringing are the ones who stop others from accessing the intellectual "property"--the very opposite of "pirates"!
In the open source world, the most direct analog of something like piracy is distributing a normally paid project for free, legally (e.g. CentOS vs RHEL). Not only is this legal, but it's completely reasonable--nobody's crying foul! (You are not allowed to use trademarks, but that's completely orthogonal to copyright.)
So really, the arguments against copyright and for the GPL are the same. If anything, the latter is actually more extreme: it ensures not only access but the ability to modify.
In the FSF's perfect world, copyright would not be necessary for the GPL--all software would just be free (as in speech) by default. So supporting the GPL and opposing other copyright is entirely consistent.