They cannot file a DMCA because there are no copyright issues.

This is why popular open source projects should seek trademark protection on their names.

Sure, people get angry about Mozilla's protection of the Firefox trademark, but this demonstrates that there are legitimate reasons to trademark a name so you can protect it from malicious operators.

Which is presumably why Firefox's downloads aren't modified (I checked yesterday): http://sourceforge.net/projects/firefox.mirror/

The GPL itself grants permission. Further, it prevents anyone from denying anyone else such permission. That's the whole purpose of the GPL.