Working with FIAT-currencies like USD/EUR will require them to use a whole new process.
To prevent money laundering and other illegal activities all purchase and sales of cryptocurrenceis to fiat requires verification of the user.

That is why on Kraken etc. you need to provide many documents to be able to trade.
Because bittrex does not have this option you also do not have to identify yourself.
They are legally now allowed to take and USD payments unless they have identified their users.