I think the point was that the GPL does not allow restrictions on fields of endeavour, and "legal purposes only" is such a restriction. Although GPLv3 restricts DRM.

I think he intended to reply the other comment comparing this to the evil issue. "This tool is for legal purposes" it doesn't restrict the use of the software (any legal use of the software is still legal, illegal uses are now just illegal in more than one way). The purpose of is to indemnify the author against claims when the software is used for illegal purposes. Whether it actually makes any difference (it's implied anyway) is another question.

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.

The copyright holder of a program can legally license their program under the terms of the GPL along with extra terms of their choosing. Of course, the program's overall license may be incompatible with the plain GPL, and the program's adoption may be hindered.

The fields of endeavor stuff is from the Debian Free Software Guidelines and similar documents.

The GPL strictly dictates what kind of "extra terms" are allowed. If the author attempts to add non-allowed terms then the GPL itself is invalidated, according to its own statements.