NOT fee free, People who Buy at the ask, and sell at the bid (Takers of Liquidity) pay, for example a 0.02% commission, while people who add liquidity, market makers, by buying at the bid, below the ask, and sell at the ask, above the bid actually GET PAID what's called a rebate of, for example 0.01%. So the exchange earns the difference, 0.02%-0.01%=0.01% profit per trade for the exchange. The 'regular' trader still pays 0.02%, but 1/2 of that is going to pay the Market Maker, effectively in exchange for taking the other side of his trade, and making the exchange he is trading on more trader friendly. aka, better.
The trader adding the liquidity, the 'Market Maker', gets earns the extra 0.01% per trade, the rebate, paid by the exchange, which in the case of opening a TON of trades, many of them ending up at breakeven, ex. you buy the bid, then notice that a huge sell is coming down the line, so you hit the bid exiting at the same price you entered as to not lose money, but you still have that 0.01% rebate n profit. Depending on how the exchanges develope, and what size rebates exchanges end up giving, rebate only strategies could be profitable as well, where you just buy the bid as huge as you can, then immediately sell at the bid with the purpose of earning as large a rebate as possible, the same at the ask, and you just do this as often as you , or your algo possibly can.
Which is great for "regular" traders because theres always someone there waiting on the bid and the ask with large orders out there for traders to buy or sell into w/o pushing the price around whenever they want. Exchanges pay this fee to liquidity providers because we make it possible for "regular" traders to execute their trades on demand, because we are there to take the other side of their trades. The easier it is to execute trades, the more liquid a particular exchange is, the better & safer a place it is to trade. Which in turn drives more traders to participate at that particular exchange, in relation to their competition. It's an awesome, natural, free market informal relationship that is good for everyone.
What I am currently trying to do for bitcoin, because I was really good at it on CITI stock before the reverse split, and virtually no one is playing Market Maker in Bitcoin right now so there is TONS of money being left on the table and/or being pocketed by the exchanges. Market Makers are an essential part of any market, and as Bitcoin matures institution and those who already have, usually professional, Prop Trading, Hedge Fund, BB's, people with institutional experience will surely swoop in and fill that currently vacant area of the market. I hope I can figure out where best to trade this strategy, learn the best way to execute it on BTC/USD, and get my size up high enough so that when large players come in to fill the void I will already have an established spot as well as a solid head start on learning and refining what is really a combination of flow trading, market making, and HFT strategies.
Hope I explained myself properly.