They use a lower fee mostly (I tried some of them before), I, too wonder how they cover that fee. I doubt they are investing the deposits or treating it like a bank but maybe they cover it as a better customer experience and from the profit they get from exchanging USD and BTC.
Well it is now obviously through their script i think they were able to discover those. Same as how bitcoin was discovered so things may be connected. The lower the fees the provide the more people would start using their system