The main issue isn't firing a customer (although it's bad), but locking up their money. It's not like they say "hey, we want to stop doing business with you, please transfer all your money to some other bank within a week or take out your money at the nearest branch", or do they?
Locking up the money is basically theft, but I'm sure the law and regulations say otherwise.
If that would be the case, won't there then be the risk of creating a short-circuit which would benefit the actual money launderers/criminals?
Say, I have 3 millions somehow deposited in the $bank_where_i_put_my_shady_money, I want to take/move the money, but doing so will raise some eyebrows, so I deliberately try to debank myself so that I can get all my money cash OR move it to $partner_bank
Locking up the money is basically theft, but I'm sure the law and regulations say otherwise.