How do you figure?
It's about politics. As President, you have to act Presidential .. you can't get down in the mud and wrestle around.
You can't go around accusing your opponent of being Nazi-like and expect to get votes. You will notice that Kerry has not done that either (he lets the Brownshirts do it for him). People will think you're a kook! And .. people might assume that Bush was saying it to deflect from the fact that Bush has been accused of being Hitler.
As a President .. he must look Presidential; not a street fighter.