The Gas Station

So, I went to get gas in my car this evening at the same 76 or conoco Phillips station I always go to and I used my Amex Creditcard to pay a the pump like I always do before I gas up. Well anyway I spent $42.45 and went home. I decided to check my creditcard transactions on line and there was a pending transaction for $99.00 from the gas station. So I called Amex and they said that some gas stations have an alotted amount they can charge you If you pay 1st before you pump the gas just in case the funds arent there. She also said when it posts, It should post for the correct amount but If not they could always just dispute with 100% protection gaurentee. So If this is legit and true I am disgusted with the control they have over our money. Perhaps It is legit and in that case, I guess It's sort of okay, But what If that extra money they chrged me "just to protect themselves" had put me over the limit with an over the limit fee. It did NOT, but still I just think It's a real screwy thing that they are allowed to do and I think in any case I will do my best to only pay with cash even though I usually never carry cash.... So has anyone else had this happen to them? I mean I've heard of banks doing this sort of thing but not gas stations, Is it really allowed???? Please let me know If you have any suggestions or info on this..