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.
I would second this as this should be marked as solved since the explanation why stake charge additional fee that will show how much the fee would be once an additional fee is needed to withdraw. I wonder what op will think or feel after reading why in op's case that the fee is $3 when in the blockchain the fee is $1. It made me think that op didn't know about some wallet provider where a person can change the fee.