The detail of why he changed his mind are his to explain, not his opponents'.
Agreed, it's not Bush's problem to explain the details of Kerry's positions. But, Bush is the opposition, the voters aren't. Why wouldn't you want to know the "real" story about his position as a voter?
I'm not even saying that one should be all that concerned that Bush is being misleading in his comments on Kerry's position. That's expected. I'm simply saying that when a candidate says something that you know isn't accurate, why do you give it any weight at all?
Bush never makes a speech that he doesn't use the phrase "flip-flop". The Republicans have learned that grabbing a catch phrase and repeating it over and over and over again is effective with voters. Why it is that people begin to believe something that isn't true simply because they've heard it a lot is beyond me. It smacks of ignorance.