Despite being less reactive,alkanes are highly flammable
In chemistry, reactive and flammable are two different term. For example, Diethyl ether is a rather non-reactive material but it is highly flammable.
In your case, propane or butane are mostly unreactive to common chemicals, but it does react with oxygen at an elevated temperature- that is they are flammable. Under similar condition, propene or butene are more flammable.