Yeah, I'm sure that it doesn't say that in the HomeBuilders Service instruction manual of how to put a dumpster in place.
From Perdue University (https://www.purdue.edu/uns/html3month/1999/990604.Kingman.gasoline.html)
One gallon of gasoline can vaporize and fill a 250-gallon fuel tank with explosive vapor; that single gallon of gasoline has the explosive energy of 83 sticks of dynamite. "Gasoline vapors are three to four times heavier than air and move about fuel tanks and unventilated areas like a fluid," Kingman said. "On a warm day, an uncapped gasoline storage tank may release several gallons of fuel.
"These lingering vapors can easily be ignited by items such as a nearby welder, water heater, or even by sparks from something such as a weed trimmer."
So, there's absolutely bone dry empty, which in that case, I'll agree with you, not dangerous, just stupid. What's the max weight that you think that gas can can hold? Fill the dumpster halfway with scrap building materials and see what happens. It'll just crush like a soda can and you're back to square one.
Then there's the definition of "empty". Is it just "mostly" empty, with like a quart of gas in it? Can you ever really get out all of the gas? It's the fumes created by a small amount of gas that's really dangerous.
