For weeks Samantha Clarke calmly listened to the insults and threats directed daily at her and her employees by people who learned they couldn't enter the Modesto, California, store without wearing a mask and following other coronavirus-related rules.

But never, says the 17-year veteran of retail sales, did she expect she'd be sucker-punched and knocked to the floor, blood gushing from her battered face. Not until it happened recently after a customer was told the last above-ground swimming pool in stock had just been sold to someone else.

"I've been in retail my whole life. I've been at this...