It has to do with the airlines as well as TSA. TSA sets the first level of rules which the airlines and passengers have to abide by. The airlines can then set a second set of rules which are more restrictive then TSA's rules. ATA is one of those airlines that doesn't allow the tanks regardless of TSA's rules.
The following airlines in the last year have allowed me to travel with my tanks:
United
Southwest
Delta
Song
American
Air France
The following airlines had an issue with the tanks and markers:
JetBlue
TSA has always let my tanks through, however JetBlue had an issue with the tanks and markers leaving Long Beach. They insisted the markers be treated like firearms (locked hard sided case inside of another locked hard sided case, no more than 3 "handguns" or 2 "rifles" in one case). The manager at the JetBlue counter had no idea about the tanks, accept she did say they have yet to have a paintball tank take down a plane and that she wouldn't let our tanks be the first ones to do it... She was confused, she thought the paint actually went inside of the air tank like a squirt gun. These airlines need to be educated on our equipment and that education needs to be passed down the ranks.
Leaving out of Tampa a few weeks ago, TSA did not have an issue with the tanks and markers, but the manager at Delta wanted all of the bags hand screened. She went on high alert when she found out there was a large tournament in the area. My bags had already gone down the conveyor belt and when I arrived in LA my lock had been cut but the tank was still inside. The bag with my markers was untouched.