When you run a stove off a propane tank, the tank is upright and your using the vapor. When you fill a GBB mag, you have the can upsidedown and your filling it with LIQUID propane.
It's significantly more difficult to ignite propane that's being fired from an airsoft gun. Infact you can shoot your pistol at a fire 3 feet away and nothing will happen, trust me, I've done it! With really dissapointing results, I was hoping for a fireball....
The fact is, if propane was that dangerous, it wouldn't be used in 90% of gas pistols.