Approved gas cans are easy to come by and not particularly expensive. And keep in mind that gasoline is very flammable, turns into a moderately nasty pollutant when evaporated, is a moderately nasty pollutant when spilled, and can dissolve an inappropriate container. Additionally, the government has a very compelling interest in keeping roads open, and preventing transportation related fires seems entirely sensible.
No one has stopped you from buying gasoline, and, sadly, no one stops you from burning it in highly polluting devices in populated areas.