Let's say I write a shoot-em-up game, where you're shooting aliens (similar to, say, Doom). I release that under GPL.
Now, someone else comes along and changes the game (which they're perfectly entitled to do under GPL, obviously). Instead of shooting at aliens, you're now shooting Shia Muslims, as an example.
They had to add new material to do this, i.e. change the pictures of the monsters into Shia Muslims. So it isn't as simple as `modification'.
Under UK law, that would be incitement to religious hatred, and illegal. Using your logic, because I allowed anyone to modify my work, I can be sued. This is clearly ridiculous.
But this wasn't an clear modification, it was the addition of new material too.
Also, this is something a bit different then just "changing" what a person thinks. It isn't even close to changing the sentence `The Shia Muslims are people' into `The Shia Muslims are FOO'.
PS. Slander is verbal attack on a person, Libel is a written attack. Given that we're talking about GFDL works, I assume you meant libel previously?
Yeah, thanks. The written word vs. the spoken one often get mixed up in electronic discussions for some odd reason.
Cheers.