And in fact, he hasn't even been informally nominated yet, nor is he the "presumptive nominee" by any traditional measure.
He has not earned enough delegates to win the nomination. He can only receive the nomination if all the superdelegates that have pledged for him show up at the convention, and then vote the way they said they would when they were polled.
They are not pledged, they have signed no affidavit, they are free to vote for anybody they want. The rules require that the superdelegates make their decision at the convention as to which candidate they think is best for their party.
As there is no way to know now how they would answer that question in August, it is presumptuous to call him the presumptive nominee. The actual ELECTION doesn't take place until the convention, and by the rules of the convention either candidate could win.
In the Republican contest, McCain has earned enough pledged delegates to win the nomination, and if any delegate attempted to vote other than how they were pledged, their attempt would be ruled out of order. McCain WILL win the vote on the floor, as there are more than enough delegates who are required by the rules to vote for him.
THus, he is the "presumptive" nominee.
He’s very much the “presumptive” nominee, and that’s why he’s announcing a 75,000 person mobfest - to make sure that the Dems don’t even THINK about nominating somebody else.