If that is the case, I would be curious as to why. Such a decision does not make any sense to me, personally. Transferring cryptocurrency is about as simple as it gets; all you need is a send address and a receive address, just like sending/receiving an email.
The only question is whether or not the transfer process is manual or automatic. Requiring a 3rd-party for something so simple seems nonsensical, especially given the fact that any manual withdrawal/approval process would most likely require action on their end anyway, as trusting a 3rd-party to handle that would be very risky and irresponsible in most cases.
Personally, I have never traded with this particular broker, so anything that I say on this topic is purely speculative with regard to how things are done on their end, specifically. I can only make general comments about how things work and/or should work, so that others can investigate and decide for themselves.
If I were trading with them, I would certainly want to know if they are, in fact, charging a withdrawal fee for cryptocurrency, and, if so, what their exact reasoning for that is. I would want to be certain, one way or the other, and be able to understand it so that I could make an educated decision going forward.