I only ever empty my bottle when I'm flying, you wouldn't empty your car tyres after use.
It's about the transportation of compressed gases
For the majority of players transportation regusltions don't apply as we are private individuals in our own cars
If transporting as a business (or using your company car/van when paintballing then it is a different matter, you then have to comply with H&S legislation and transport regulations, eg making sure the insurance is valid, the correct stickers are displayed and cylinders are secured
Paintball cylinders are usually too small for anyone to really worry about, but the fire brigade would prefer that we had the compressed air symbol displayed. This lets them know what that cylinder is when they discover it halfway through cutting you out on the motorway