Non-warranty clauses also override legal mechanisms to favor the copyright holder. So what? They don't impede the use you can make of the software. But they are uncomfortable, because should damage arise following use of the software, the user won't be indemnified. In these cases, non-warranty clauses undisputably harm users, likely more so than choice of venue clauses. Yet we (rightly) don't consider them non-free. The same reasoning should be applied to choice of venue clauses.
Non-warranty clause are illegal in Europe. However the warranty applies only in the case of commercial transaction. I am not sure you can claim any warranty for a software that you have downloaded at no cost; wether it has a non-waranty clause or not. But it is true that if you sell a GPL software (or other free or non-free software); you must provides a warranty (at least in Europe). This warranty cover only "what is normally expected for the software by a normal user": it does not say that you can sue the person who have sell you the software for any bugs it may contain.
Olive