IANAL but my layman’s understanding is no, there is no law preventing them from halting withdrawals and a temporary suspension can on rare occasions stop a bank run. Problem is, clients can sue to get their withdrawals and the judge in charge of the case can force the business into involuntary bankruptcy proceedings (without the protection of voluntary bankruptcy) if they think the bank can’t meet the withdrawals in full, to get a valuation of its assets so that the plaintiffs can be made whole. This won’t happen over the weekend because courts aren’t in session but can happen on a week or two timescale, in which case the business is basically frozen for months while everyone fights over a half dead carcass. It’s a relatively catastrophic outcome for everyone involved.