Mechanic refused to rotate my tires because the tread was too low

Most states make the tire illegal to use on the road at 2/32 inch tread depth. That’s when these little wear bars are flush with the tread. The picture below shows Barely legal tire tread. The bottom line; if the tires are at the wear bars, even if the state doesn’t say they are illegal, they are not safe to drive if it rains.