Some cops would have issued a written or verbal warning. An expired registration does not make the car any less safe does it? I can see it differently if the insurance had expired.
My friend had his triaxle red tagged by DOT on I 81. He had to have it towed, but when the tow company showed up, they said they did not have to tow the truck but could just drive it to their yard because they are an official inspection station. That makes no sense to me, if the truck is unsafe.