They all say that and have enacted their rules (not laws) because people abused their return policies, ie: they used mics for the 30 day return period and returned them.
It's a shame they had to be pushed into it, but I do understand.
Some people ruin everything for everyone.
Any store, even those who have this rule, will work with you if they know you and know you're a serious buyer.
What if you just want the mic to record acoustic guitar, where's the health risk?
There is no way to prove a mic has been at your mouth. Besides, who uses a good mic without a pop-screen (at least condensers)?
It's all rules enacted to stop a-holes from abusing return policies.
As for the law - ask where you can find the law.
If there is one, it must be published and available online.
Let's see it.
Still, even if there is one, like I said, all you have to do is say the mic is not going to be used for vocals.........and you promise not to stick it in your ear.