The construction of a NASCAR car is different from ours. We have a real car, with real strength, with a cage inside it. They have a roll cage, with a car "body" (with zero strength) stuck on the outside. In addition, since their cars are much bigger, it could be several feet to reach the nearest "body" panel. Their rule makes perfect sense in their context, but not in ours.
Not exactly. Tube based cars like a late models and above are much more substantial than a Miata without the body panels. The body panels do very little for impact. I'd bet either my 83 Delta 88, '80 Impala or any of the late '70s Montes I'm working with now are much, much more robust than that Miata tub that me and one of my helpers can move completely stripped. The mini stocks don't have to welded to the cage but the hobby stocks and street/super stocks do and they are based on production cars. I wouldn't build or work on a stock car cage where the seat wasn't tied to the cage. I've looked at do it in my 90 SM but it's tight with me in there. Were I not so tall it would work and now that I'm down to 250 (from 310) it's easier to get in and out. FWIW I raced stocks at about 280. Anyway, I'd strongly recommend that any race car guilder, regardless of make or class in a saloon type vehicle make the seat mounts part of the cage structure.