How do they get away with it? By being the 400 pound gorilla in the situation. One can infer that someone at Wells doesn't know the law, or that he or she (or higher-ups in the organization) think they can get away with it simply because it's a big organization with more resources than the average bank. I won't offer my opinion on that question.
You indeed have the choice of charging the check back to your customer, who may or may not know what the UCC says about a bank's refusal to honor its own cashier's checks, or pushing back on the check issuer yourselves (in which case you'll have to either put the amount in suspense pending resolution or try to re-present the check hoping that the issuer will come to its senses the second time around).
I'd start, perhaps, by trying to contact Wells and whatever department is responsible for returning the check. Sometimes, when the law is spelled out, especially when an employee's mistake is involved (rather than the 400-pound gorilla), issuers have been known to reverse course and agree to pay the check in question. If that gets you nowhere, you'll need to decide whether you or your customer will have the pleasure of talking to legal counsel.
Of course, if your customer doesn't have funds to cover the charge-back, your options are fewer.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8