If you're so concern about the fee you should have think about it or read their TOS or any information regarding their site before using their service.
They do mentioned they will charge additional fees before to withdraw on TOS (as posted above $3), but they didn't give full detail between the fees that be paid to miners and additional fees to proceed the transactions.
Many people already give an answer, even @Stunna has explained the reason why does they charge particular fees... but @OP didn't comeback or even log in to continue about this matter. I think this case can be marked as solved due to no one complaining anymore.