On the contrary, doing this is practically a requirement for being a bank. US law requires banks to file a suspicious activity report if they see unusual activ

On the contrary, doing this is practically a requirement for being a bank. US law requires banks to file a suspicious activity report if they see unusual activity in your account that may be associated with money laundering or criminal activity. What qualifies as suspicious activity is not well-defined, but can be as simple as large unexpected transfers into or out of the account. The law also requires banks not to tell you about it. They don't necessarily have to close your account, but most will, especially after several reports, because of the risk of failing to comply in the future. And since they're not allowed to tell you why they closed the account, you end up with opaque reasons.