It is pretty well documented that AM General views their vehicles as something they sell to the government and they wish them not to be in private hands. Try calling them about anything, regardless of how trivial it is and telling them you're a civilian owner of anything they've built at any time and see what reaction you get.
Since I don't know their motives and have to assume, I'd say they see road use by civilians as an opportunity for lawsuits and PR issues. Some doofus buys a 1993 M35A3 and drives around on 25 year old tires, front tire explodes, trucks hits a school van and kills a bunch of kids. You think AMG isn't going to get sued?
If I was AMG, that's sort of how I'd view this. Sure, you shouldn't drive around on 25 year old tires but AMG doesn't have any control over that. Sure, the lawsuit will be dismissed as frivolous, but they'll still have to spend the money on getting it thrown out and the PR issue will still be there.