Brian, the reason dealers usually have a limit is b/c they, just like all merchants, are charged a fee (usually a percentage ie: 2.5%-4.5%) by the company that pays them and collects the money from the credit card company. So, an $18000 purchase would cost the dealer an extra $450 - $800 fee that has to be taken out of the gross profit of the sale. So you say, why cant they just add that fee onto my purchase so that i could buy this car, get a great rate and also earn sky miles all at the same time? the reason, all merchants sign an agreement with their local company who handles credit card transactions, not to pass this fee along to the customer.
So when you stop and think about how much money credit card companies make in interest alone from these poor fools who carry high balances, they also make a portion of that fee that is charged to the merchant.