A 10 gallon fuel cell in a race car or a 1 gallon portable gas tank sitting on a boat aren't the same as a 500 gallon homemade bladder stuffed into an overloaded passenger car. I honestly can't tell if you're being facetious here. Also, a guy bringing gas home for his lawnmower is doing something useful that justifies the risk somewhat. These guys are purely motivated by greed. There's no good reason to drive these trucks around.
-These trucks have no HAZMAT signs to alert first responders to the dangers within.
-There is no separation between driver and cargo, so things like smoking or even electrical sparks could ignite the gasoline.
-These bladders don't appear to be commercially engineered or fit for this purpose. They could easily leak.
-The trucks pictured are running well over their rated payload capacity and are 10+ years old. 500 gallons of gasoline weighs 3125 lbs and even the large Excursion shown is only rated for 1500-2000 including driver and legitimate fuel.
-The drivers are incentivized to run away from police at the scene of an accident and their insurance won't cover criminal activity. If a commercial fuel truck catches fire and someone gets injured due to burns, they'd have to pay.
-The drivers aren't properly trained for handling hazardous materials. They could do things like stall on railroad tracks and kill hundreds of people.
To me, the risk seems much higher than a hobbyist with a custom hot rod or a legitimate tanker truck.