Just so you know, I voted John McCain, but I will try to make my answer as non-partisan as possible.
SNL tends to be a more democratically inclined program, most producers just tend to lean to the left. SNL also seems to target the younger generation, usually kids 16-25. You probably know that most kids in that age group today tend to have more non-partisan and liberal idealisms, and that is just why SNL seems to target them. Although, SNL never takes up the chance to let someone famous or well known on the show. They asked McCain (actually, over the past year, every major candidate showed up on SNL) if he would show, and he quickly obliged. Humor is one of the best ways to dive deep into someone, and that is exactly what McCain strived to do. It is just another tactic to get peoples support, because you know sure in hell not many kids even watched the debates!
SNL bashing Sarah Palin doesn't affect peoples' votes. Any straight-minded person isn't going to base their political ideals on something grotesque and negative such as that, its not even the real Sarah Palin! (except for one episode, I know she made a guest appearance.) When someone sees a REAL presidential candidate on something as zany as SNL, it connects with people on a more personal level, and subconciously, people will tend to gravitate whoever can make them happy (or laugh in McCain's case!)
Basically, you liberals would call it propaganda by John McCain I guess. But remember! Every candidate can AND DOES his fair share of troublemaking while running!