>"Won't propane or natural gas work?"
For an liquid-liquid extraction (of the THC oil into the liquid hydrocarbon), propane would have to be under very high pressure (like in a propane tank!) and methane at an impossibly high pressure for a DIY set-up. Butane needs very little pressure to contain it and when enough flashes to a vapor, will be at liquid at its boiling point (30F-ish, depending on purity, isomer, etc).
I've never done that particular extraction, but I suspect pentane would work just fine and be considerably less volatile, especially if you kept it cold. Still dangerous and still best done in a well-ventilated area. But you can't buy containers of pentane at Home Depot. It makes me wonder how toluene would work, since it is readily available, far less volatile and cheaper per pound than butane canisters.
The beauty and the danger of butane is that once the extraction is complete, you could just let the butane flash to a vapor and have the extracted, pure oil remaining. Toluene would have to be boiled off and this demographic obviously has enough problems with flammable vapors and ignition sources already.
I liked the quote in the article: "I call it the exploding hippie phenomenon."
Since it is a simple procedure and there is a quite a danger working indoors, I wouldn't be surprised if some people starting performing this extraction in the wilderness or at least at the end of the road. So if you see a pile of butane canisters, now you know why.