[jasonmunro]

While it's true that stock exchange operators are private companies, they are also SROs that undergo significant scrutiny from various government organizations (FINRA, SEC, etc).

Most (all?) exchanges use "maker/taker" pricing, so while it is possible the trading platform might not make money on _every_ trade depending on the exact pricing rules, they do make money in the aggregate, and in general more trades equal more profits. Many of the opponents to HFT believe that the maker/taker pricing model is a big problem in the markets, and doing away with it would significantly curb HFT profits. Personally, I'm ambivalent about HFT. All stock trading seems to me far removed from the actual value of the underlying companies anyway :).

Disclaimer: I was an engineer at BATS Global Markets for 5 years, but have been out of finance for the last 4, so my info may be out of date.