You've taken the quote out of context, what he actually says is "This tool is for legal purposes only". That's quite clearly intended as a restriction on its us

You've taken the quote out of context, what he actually says is "This tool is for legal purposes only". That's quite clearly intended as a restriction on its use. The problem being that if the user does something illegal in a very minor way then he's opened himself up to a potentially large copyright suit because he's violated the license. The point of the GPL is to stop the copyright holder from dictating what you can and can't do via the threat of a copyright suit. Extra clauses like this go against both the letter and the spirit of the GPL. Either his extra clause is nullified by the GPL, in which case he's only created the illusion of legal indemnity for himself, or his extra clause is accepted, and the GPL is rejected, in which case he looses all of the important legal protections such as the warranty disclaimer. Either way his attempt to protect himself has failed.