Even with overnight margins there is room for play. Officially margin is imposed by the Exchange Clearing Corporation on it's member firms. The sum of margin funds posted by those member firms is based on NET positions held. For example if IB has 5700 contracts held on the long side by customers and 4700 shorts (obviously by a different set of customers), then IB need post margin only on 1000 contracts (5700-4700=1000). While the exchange isn't crazy about under margined customer positions, it ultimately only requires that firms make good each night on net. The total deposited margins that are not needed as collateral to the Clearing Corp. are a great revenue center for member firms.
pabst. I believe you are mistaken. I'm almost certain the states has the same rules we need to follow in australia and hong kong. If we have a net postiion of long 20 and short 19, we need to inform the exchange of this position and pay margin for 39 contracts. as for firms charging different day rates, one thing to consider is the risk of someone putting on a large position and blowing out the entire firm. This should be a genuine concern as it has happened on more than on occasion.