By who? If it's by internal rules, they are not constrained, they choose not to share info.
probably whatever legal investigation is happening about the real source of the funds.
You're not allowed to tell users that an account has been flagged for criminal use, as this tips off the user and makes you an accomplice.
Then just let the account run? Anyone actually using their account for criminal purposes is not going to be surprised their account is blocked, and will have a very good idea of why it happened.
If you suspect criminal operation you have to shut it down. What you tell the customer is up to you, but if you're right and you tipped them off then you could get prosecuted as well.