@robryk Because empty doesnt always mean empty... A tank can have gas stop coming out and then when you turn it off pressure will build up again as it warms. This is particularly true if they are a liquid at pressure.
So it can read as empty and gas might not come out but when you go to puncture it it may have re pressurized by then.
The thing that caused me to wonder now was a shaving cream can, where I could test the pressure inside by deforming it with my fingers first.
I also saw such a warning on camping gas bottles (some mixture of propane and butane), which did have a way of ensuring they're empty provided: there's a plastic shim you can push into the valve to permanently open it (so that you can vent the bottle after you used up the gas). In such cases I don't really see a reason not to amend it to say "don't puncture unless the shim has been inserted for a few minutes".
@robryk butane is particularly troublesome because its a liquid at pressure.