Do they? It's by far the largest BTC exchange - by a factor of ~50 or so. If you want to exchange more than a couple hundred BTC, they're the only game in town right now.
Please don't conflate the free market with anarchy.
You should read up on the foundations of anarchy before spouting your mouth off, since you are woefully ignorant on the subject. Bakunin, Kropotkin, and Bookchin are all good places to start.
Coercion does not have to be done by a governmental entity. The philosophy of government can wax as poetic as it wants but I know too many people who would happily take advantage of anarchy to employ said coercions. Practically speaking, Human nature being what it is, coercion will be rampant in an anarchic society. I'm quite familiar with the foundations of anarchy. I just occasionally use poor wording :-)