As mentioned, this is a very old rule, long predating TSA.

Yes, a never-used container would be harmless, but there is such a vast variety of camping fuel modules (and fuels) that this policy was probably meant to spare gate agents memorizing every variation of stove container—or missing one. Ozgal's container was most likely confiscated because it was a type meant specifically for flammable fuels, not food. (Yes, one could sneak camping fuel in a drink bottle: once upon a time passengers were considered generally trustworthy.)

Exploding shampoo aside, some precautions are warranted. Fire on aircraft is even more dangerous than on ships at sea; you can't just pull over to the shoulder and get out. Coleman fuel, for example, has about the same volatility as gasoline/petrol. Although it may usually be carried without incident, it increases the risk of a bad event cascade significantly. ValuJet 592, probably TWA 800, and Swissair 111 all were lost with all souls due to unexpected, uncontrolled fuselage fires.