This sounds like a recurring urban myth to me. The porta potties are vented thru their roofs. This oldie was explored on myth busters in regards to regular toilets and proved impossible.
Just tossing a lit butt in the gas would not ignite it. They had to use a lit match to make it go off. Either a weak brained investigator using an urban myth to spice up a slow news day or an anti smoking nazi on a jag.
My money is on the latter!!!!
Actually that was after spraying Lysol or an air freshener. But IMO this is bogus their is a vent pipe from the waste container under the seat to the roof, no way for gas to build up.