okay i didnt read everybody elses responses but here is the bankers explanation:

the bar is going to preauthorize a higher dollar amount because before they run a tab for you, they want to make sure you can actually pay them for it. most places just use a standard number, such as $100. if your tab was starting to run higher than that, chances are they would have reauthorized your card. hotels usually do a similar thing to make sure you dont destroy a room or incase you steal towels or rent a bunch of porn.

gas stations are different, they typically just run a $1 authorization to make sure the account is open.

more banking law: if a bank authorizes a transaction of any amount, they are required to honor the authorization. for instance if you get gas and they authorize your transaction for $1 and you get $50 worth of gas but only have $20 in your account, your bank has to the transaction for $50 whereas if you had written a check, the check would be returned NSF.