Generally new accounts claiming promotions will receive the same accusations/multi account so do you claim the deposit bonus or freebet? The first time I created an account and requested my withdrawal I got the same accusation but since I didn't claim any promotion so I can prove there is no reason to have more than one account and that reason is acceptable. I have checked your thread and they give back your deposit while all your winnings are frozen, yeah we don't know if that's fair enough but they don't take your money it's just that your efforts to win it are in vain.
It cant be general, casinos should never do such a pratice although new players take bonuses on their first play because it is not fair to players to be accused something that they did not do.
Accusation should come with solid evidences including the one experienced by above user, FJ need to prove the accusation that he really own multiple account or breaching other terms that make him lose what he has won.
Getting his initial deposit can be something fair if the accusation is valid and proven that he breached the terms but if there is no evidences, I dont think it is fair to confiscate his winnings.