Well, confesticating winnings is different; holding the deposit is different.
I believe, if there was abuse going on; the best move BC should do is to give back OP his deposit and ban him permanently (if he is proved to be guilty of doing prohibited practices such as multi-acc).
Holding the deposit should not be in the power of any casino in any matter.
Not unless the balance itself comes from the profit of the previous action which is the abused on sports betting which I believed they have grounds to confiscate since they already established the connection.
I will agree to you if the OP is just being restricted for violating the terms that doesn’t involved multiple accounting connections that made a profit already from previous account on same casino.
If OP is in profit, yes your argument seems better.