Jason - American Express issues charge cards and credit cards. The charge cards are to be paid off monthly. The Amex credit cards work just like a Visa or MasterCard, you are charged interest for any unpaid balance. A hold or block would not incur any interest charges. If a deposit is charged to your Amex credit card account, and you carry a balance, you would incur interest charges on the deposit.
Sixtisbest - Correct, with the major US car rental agencies, you are renting a high dollar vehicle WITHOUT a substantial deposit or hold. BTW the car rental companies generally only have leaseholds on their vehicles. They don't "own" the cars.
I don't mind a hold on my account, and I will specifically ask that NO deposit is charged to my Amex credit card. I frequently carry a balance.