I noticed that and wondered that too. I talked to some guy at a old antique tractor pull and he was using a late 1960's or early 1970's cabover Freightliner to haul his tractors around and he also had antique plates on as well. I asked him about the antique plates,and was told by the notary you can't use that vehicle to haul anything. His reply was there are ways around it and just need to do your homework. My thought was it's who you know and who you blow.
I will probably get antique tags for my big truck anyway when I'm done with it.