Well then why the hell do they use them? If they're guaranteed to start structure fires! Or is that the whole point, in last night's case.
And why did the fire start so long after the initial use of gas canisters? I'd have to check the time stamps between "gas has been deployed" and "fire reported" but it doesn't stand out as an immediate chain reaction in my memory.
They use the gas to drive the suspects out of where they are hiding to protect the cops, but apparently nobody has figured out how to make the gas come out of the cannisters without generating heat. As to how long it takes for a fire to start, it would depend on what the cannister landed on. If it landed on a piece of furniture, quick fire. On a tile floor, no fire.
Fired through a window and bouncing around who knows where or on what it would land.