I suspect they don't need to expose themselves because they are holding large amounts of both Bitcoins and cash, so they can simply sell them to people at whatever price they want to. It's not like they take your USD and then have to run out and buy Bitcoins to cover them. They do have to worry about the value of their Bitcoin and USD holdings becoming worthless, but that's something all Bitcoin and USD investors have to worry about.

If they are taking some risk, by making commitments that they don't have backed with currency, it's trivially easy to calculate their exposure and figure out how high their fees need to be to cover it. That's a problem actuaries solved decades ago. Currency exchange is not a new business.