Well it's a lot more complicated than that.
Most states adopt fed dot regs, some will add or modify a bit. So it really doesn't vary state to atate.
But fed dot is fairly plain. Under 600 lb product can be horizontal, over 600 lb product has to be transported upright. Product weight doesn't include cylinder.
For at least 30 years, different states have been writing those tickets to people who have way less than the 600 lb of product on board. I've seen the fines as high as $2000. Those tickets won't hold up but they make quite a bit of income before it all shakes out.
Cylinders can go down the highway, legaly, with gages on. They just have to have a cover or be in a cabinet. MSHA will let you drive with gages if you have a cover.
But dot is only one agency to take into account. OSHA and MSHA will both fine for horizontal cylinders. So even if you're legal on the road you can get hit on site. There are quite a few sets of rule$ to take into account.
Most large GC's have their own hid deep in the safety manual, a lot of project owners have their own. Those can differ from both dot/msha/osha.
I'd agree that the rack in the pictures would bring you all kinds of grief in a roadside visit with a dot cop.
I have driven by DOT with this setup. I agree the cylinders should be secured higher up, but the ARE secured with this setup. DOT looks for them laying down, and not secured.