A commercial vehicle will be subject to OSHA regulations (covering businesses) and DOT regulations (covering interstate or potential interstate commercial transport) as opposed to Joe Diver, who only has to meet their own state DOT rules.
Partly, those agencies don't expect a random "inspector" to understand which tank might or might not explode, so they just want them all secured. My friend's father was killed by the spare tire, which was unsecured under a semi trailer, and cam bouncing across the highway and through the windshield--and the driver. Freak accident, but that's why there are regs.
Personal drivers are pretty much allowed to do any kind of foolish thing they want. On the one hand, many of us store tanks in the trunk or cargo area. On the other hand...storing them in the rear seat floor puts them further inside the protected are of the cabin.
Sometime after you've crawled or walked from what used to be a perfectly good car, and things have quieted down, you may notice that anything, literally ANY THING, that was not bolted down or secured, had become a missile, and that even something like a six-pack of soda cans can punch a nasty hole in the windshield. Or someone's head.
This is why airline crew ask you to remain belted in your seats. Most folks just don't appreciate the reason why, until after they've gone bump.
