If you have a truck frame that needs welded then you can weld on the truck frame if you want too and there's nothing illegal about welding on the truck frame.
Truck frames are made of metal and they can be welded and the factory welds on the truck frames when they manufacture the truck frame.
I've fixed broken and rusted through truck frames before by cutting the bad spots out and welding in some more metal to repair the truck frame.
As long as you can weld properly and the weld is done correctly so it won't come apart then welding the truck frame is perfectly safe.
Trucks cost a lot of money to buy new and used so if my truck frame gets rusted through I'll repair that truck frame and keep the truck on the road.
Unless the truck frame is completely gone then there's no reason to scrap the truck out because it can be repaired.