In addition to not knowing the true economics theory, those sites' business models are different.
Some sites, for example sites like OKCoin, treat BTC as commodity and provide many tools to help you trade BTC against fiat currency. They need the users to deposit some fiat currency with them as the margin trading purpose. Those sites still want to make fiat currency profit from users by being the exchange or middle-brokers; being the middle man of the buy side and sell side, those sites make a steady risk-free profit in fiat currency. You can not have business with them if you don't contribute any fiat currency to them at any point.
Some other sites, for example sites like some gambling sites or BitMex, treat BTC as the base currency, but they need you deposit some bitcoin as the deposit or initial margin to them first and want to make BTC profit from users. They will calculate users' profit and loss in their private system and therefore users are facing sites' credit risk where these sites could be in trouble and the users can not withdraw their whole profit.
However, in this site, it treats fiat currency as commodity and does not require any initial deposit or margin from the users. This site doesn't make any profit in fiat currency or BTC. The users have no any profit-and-loss relation with this site, they won't be afraid this site shutdown while they are much afraid of the shutdown of those sites with their margin profit vaporized.
Here in this site, users will make profit-or-loss in BTC by their own P2P business and convert the profit-or-loss into fiat currency at their own discretion and using the bitcoin network as the money settlement infrastructure and using multisignature address as a way to avoid trouble from the deal counterparty.