Basically the dealer wants to make as much money on the deal as possible, so he wants to avoid paying the credit card company. Otherwise there is no reason not to take it. Don't believe me, ask him for a free sunshade ($40 retail) and he'll likely say no. Cheapo.

Using a credit card for a purchase (like Quicksilver said) is a great idea, just don't expect a dealer to let you cover any significant portion of it. I've never heard of them going more then $5,000 - $6,000 max.