That's why it's harmful. If it wasn't allowed by the license, it could simply be prohibited by enforcing the license. If you don't think the outcome (as I described in an extreme form) is harmful, then we just have to disagree. I think it's harmful, so I don't like the FDL.
That you consider it harmful is one thing, it doesn't make the clauses harmful in it self.
In theory, anyone can go and make Linux a non-free program, since it is simply impossible to enforce the license there.
Back up this claim.
What part of `in theory' didn't you get?
Cheers.