Red Hat's warranty doesn't provide for a legal defense, but it does reduce the risk of losing a suit. Red Hat promises to modify infringing software or replace it in whole with a copy that doesn't infringe.
Really, it's a long shot that SCO will go after the Linux community. To run afoul of the Digital Millennium Copyright Act, you must commit an intentional violation. That's hard to do if you don't even know what portion of the code infringes SCO's rights.
HP, Novell and Red Hat are simply assuring their customers that buying and running Linux is safe. That's golden, but what glitters is that these companies are stepping up to the plate and taking responsibility for the intellectual property in their distributions. That is a big step for open-source software like Linux.