I do, since it happened to me 9 years ago when I was as naive about the industry as you. First National Bank of Omaha, the largest privately held bank with $17bn in assets, if you're curious. The several thousand dollars of customer payments they had not yet disbursed was held for exactly 180 days before they released it to me.

Do you not recall because you haven't read your own agreement, or because you haven't actually opened a merchant account with a bank before? You don't have to take my word. Type ["merchant account agreement" 180] into Google to see some 40,000 examples of bank contracts with that same hold period written into them.

I've worked in a bank, and we did freeze accounts on money laundering suspicions - it's not even a choice, the law requires to do so in certain conditions.

And the OP case description is quite unclear, but the details sound like that it might be not "assets frozen while check if business is legit", but actually a threat of "charges filed against owner for circumventing anti-money laundering laws and assets confiscated".