Confiscation is nonsense....... Yes, it would be way over the top to ask store employees to participate physically in addressing theft. They are sales people - not law enforcers - and are only trained to function within that scope.
When a legitimate consumer calls AT&T about a new or existing account, it is standard practice for them to acquire information from the caller for security reasons. Why is it such a big deal to include IMEI information to ensure that part of the security issue?
I worked 12 years at a cellular company and I know all too well how it used to be, when trying to activate a new phone. You needed the sim card number, as well as the IMEI number. You either had a match or you didn't. Having said that, all that should be required of a store(salesman) employee is that they kindly inform the customer if the two do not match. There's no need for them to wrestle the suspect to the ground trying to confiscate the phone. That's just ridicules.
They should have enough of the customer's information to direct the proper authorities appropriately if that's an option the store or law enforcement wishes to pursue.