Agreed, although you might be able to force Travelocity to find the same routing by playing with their multi-city settings.
As it happens, just moments ago I booked tickets through Travelocity via Ultimate Rewards Mall and paid with American Express. The Travelocity site defaulted to use a stored Chase Sapphire Preferred card and the button to Enter Another Form of Payment seemed to be greyed out. Nonetheless, the transaction was processed on the American Express card. I do not know if this was a coincidence, but I will post again when the Ultimate Rewards Mall bonus is posted (or not).
I am looking to get airline award tickets issued and want to use a different card to pay for the additional amount due - is this possible?