Because a responsible journalist having reported that Obama campaign spokesmen made this claim about McCain should follow up and attempt to verify it by putting in a call to McCain's office.
Why does no response (yet) signify that the claim is true but doesn't signify that it is in fact still unsubstantiated?
Try to think in terms of a skeptic evaluating any other claim. (Try to strip it of the politics and partisan considerations.) We have a claim made by someone who was not privy to the information. So far, we have see no substantiation of the claim by anyone who was privy to it, and we also have no refutation of the claim by anyone who was privy to it. I think it's irrational to read any significance into that silence at all. All we have is an unsubstantiated claim from someone not privy to the information.
And FWIW the nature of the claim has problems of itself. It requires thinking someone (McCain) acted starkly inconsistently.
I still reserve judgement and consider it simply an unsubstantiated claim. It could be true, but so far I reserve judgement. The longer we go with my request for a reliable source going unfulfilled, the more I begin to doubt it--but it's early yet.