It is for OUR safety. Tanks typically fail during the fill process. You may trust your tank, but my staff and I can't take that chance.
We have people bring in paintball tanks that have been improperly repaired, abused, misused, or illegal. Our only protection is our visual. We inspect the outside of the tank for damage. Check the hydro date is valid. Check the burst disks for proper pressure and for the minimum number of vent holes.
We require two vent holes for safety. In the first two months of 2009 we had four (4) paintball tanks blow the burst disk during fill befreo they reached rated pressure. Three of these were brand new, from the vendor, getting the first fill. If a burst disk has only one hole it will spin like it has a rocket motor attached. For the staff this is incredibally dangerous. The last single hole burst disk we filled blew while attached to a fill hose. It spun around and picked up six (6) aluminum 80s (weighing approximately 250 lbs) and spun them around, entangling the hoses attached to those tanks. If a staff member had been near I have no doubts there would have been injuries.
If you consider our requirements to be too onerous, please feel free to have your paintball tanks filled somewhere else.