--snip--
turns out that Freeze 12 seems not to have propane or other flammable component, but there are good reasons that regulatory agents rule against a product and that manufacturers recommend that things be done in a certain way.
--snip--

Freeze-12 doesn't have propane but according to the MSDS the R142b component is "extremely flammable" and breaks down into some pretty nasty gases due to "thermal decomposition". 
Freeze-12 is on the EPA's "acceptable subject to use" list, as well as some others that have less than 5% isobutane. Perhaps they feel that at level the explosive potential is low enough to be acceptable? I haven't seen where it was illegal in some individual states; does anybody have a link to that?

I guess you picks yer poisons...

Cya,
Robert