This is what I do consider cooperate fraud because I don't want to believe  the Admin of such platforms aren't aware their customers are been charged that much  and to even tag it mining fee for USDT gives me more reason to consider it skeptical and very unclear.  The fact that after your complaint such was lifted off your account and you don't have to make such payments mean while others may be paying such is another reason not to trust that platform, I mean if they have a policy for withdrawal it shouldn't be selective in a way they get to take advantage of their customers due to possible lack of knowledge about  the process and what is applicable.

If a withdrawal fee is to be charged on a transaction then it should be as transparent as possible not having some persons paying and the others probably withdrawing for free, most likely there may be other activities going on that you may not be aware of because if they can do such then I don't see the possibility of them not doing  more, make sure your data is safe with them and be careful to make huge transactions on such platforms, personally I will advise not to use them any further just to protect yourself from anu issues in the future their  operational terms isn't clear.