I have used them forever as well. Using one now on my 2013 for diesel fuel. I think you will find that it is referring to gasoline. There is a legal limit that you can only have as well. It's a small amount but somewhere around 100 gal and you have to have placards.
You also have a fume issue to think about. Your main gasoline tank vents the fumes froze the evaporated gas out and into a charcoal canister that scrubs the fumes. There is also a requirement for a rollover vent on new vehicles. A aux tank won't normally have that and that's where the EPA Nazis come in, that's my bet.
Like I said. Just by bet on the subject. I'm gonna do some digging on this thru my industrial safety resources because now I'm couriuos.