Well it's pretty obvious MA is not a bank (at least as long as they don't have banking license).
They are ordinary company that is emulating virtual currency which is tied to USD and freely convertible both ways. From the customer's viewpoint it's important that the system works as promised. In every practical aspect, that's all that matters.
Which dollars exactly are they using to pay salaries? Who the hell cares as long as your withdrawal comes through? Suppose one day it won't come through any more. Is it important then? Well not really. Either way it's just useless nitpicking that doesn't change anything anyway.
In the eighteenth century it would have worked like this: u step into MA office to hand over your $100 in cash, having carefully written up the serial numbers on the bills. Then few days later u appear in the office again in person for your withdrawal - and demand that they give u the exact same bills. Those were MY bills, my own personal bills, understand!
If u try that today they'll say We're terribly sorry sir, but we have no idea what u talking about.
Neither do I.