Definitely responsibility reasons, but I don't believe it is a law most places. Tires which have worn differently will wreck havoc with differentials, even open diffs. Tires which have worn differently can make for weird reactions when installed on the front of rear-wheel drive cars, making poor road conditions dangerous. Of course, the same is true of tires that are under-inflated. I see so many vehicles with one tire that is half flat...the highway Patrol must see them, also, but they don't seem to mind.