I too believe that is the answer...
They know what I am finding out... bronze guns, especially repros, can be pretty crappy internally (like I said, if I get a porous and bubbly mess of a barrel casting, I can go over it with the TIG torch and make it look great... on the OUTSIDE... but it is (or can be) a bomb casing underneath). So if I am a sponsoring organization and have to go find liability insurance to cover my events... darn right I am going to require liners. The only other option would be for the sponsor to hire an NDT (Non-Destructive Testing) contractor to be at the meet, out in the "south 40" with a lead encased Cobalt 60 pill, lots of film and a developing shack to assure me those UNLINED bronze pieces are safe and will not kill someone... especially if it might be me!
On the other hand, if I have machined my tube from solid bar stock and TIG'd on the trunnions, I would be inclined to stay home from shoots requiring me, with my high-integrity tube, to have a liner. I'd go find some other rednecks to shoot against (and possibly let one of THEM kill me). Maybe you can't win but by building garbage and lining it!!!!!!