Because we will always remember how he signed a law that would be unconstitutional and how cavalier he is about giving away our free speech. This also goes for McCain, and all the others that voted against our free speech.
He may be trying to defuse a possible plan to use a veto as proof that Bush does not want campaign reform - look how he got into office - with the Supreme Court. But, this goes against the trust we have placed in him and in his word. It will take a long time to rebuild that trust when torn down.
My firm belief in Bush and his goals has been shaken by this action - I am disappointed.
You can be sure, however, that George Bush II is no Ronald Reagan.