It is a legitimate question. I think the answer is: because the president gets to set the terms for this interview, and that's what he wants. He is doing this as a favor, and he doesn't want to set a precedent of having the president or VP being hauled up to testify for every Democrat commission that comes down the pike.
Also, this isn't a mid-term exam, and it's certainly not a legal proceeding. It's allegedly an attempt to find out what happened, and how to fix it. Bush has a lot on his plate, and it's helpful to have somebody like Dick Cheney there to help him keep track of what's going on.
So the commission gets to talk to both of them, but on the president's terms.
Because the Dems are only interested in finding one flaw, one mis-spoken word, anything to get a simple headline that will resonate and effect 5% of the voting public into questioning President Bush and possibly switching their vote. That is why they want to 'divide' the POTUS/VPOTUS and manufacture a controversy or discrepency...
This entire process is being orchestrated by the DNC/DemCongress-Senate leaders...by the way, do you think it was just a coincidence that all those reporters asked virtually the same questions over and over last night?
They were all under "orders' via a coordinated strike effort.
Know your enemy...