Not that it stops anybody, but they are illegal to transport them once you refill them (the disposable one's). This is a direct result of slob campers that do not properly dispose them, or simply leave them around the campground. Hell, I remember going to a campsite after summer ended. The entire campground was covered with 1 to 3 feet of trash (I'm talking about 30+ campsites). I camped further in, in a spot with little to no trash. But on my way out I stoped and pulled out trash bags and began filling them (an entire box). I filled my bed to the top of the shell with full trash bags. In town I stoped behind a business center puting 2 bags in a bin so I didn't fill and inconvenience any business. A Sheriff deputy saw me and pulled up, he intended to cite me until he looked in the back of my truck, quickly said have a nice day and was gone.
The issue is a people problem. Local to me the campgrounds, day use areas... are littered with trash, dirty diapers, and yes, empty propane cylinders. I certianly wish they'd cite the offenders. But it's easier for government to be big and impose regulations.
I don't like it one bit, unfortunately, it isn't going to change.