Agreed, there's enough drama in dive communities already. Try addressing it with him. Do another dive with him, and maybe catch him off guard. If he does it again, ask him why he backed off. Let him know you were trying to practice a skill/some skills like all good divers should.
Having said that, I agree that a single tank to 1/3 is WAY more dangerous than two tanks to 1/6. I think one reason for the expiration, is that a lot of divers will get complacent. I can see this happening: "Well, I have 20 safe logged Intro dives to sixths on doubles....so maybe I should push a few hundred more PSI. Apprentice is like the same thing, but more penetration....why should I waste my money? I know what I'm doing! All I have to do is turn at thirds!!" Then, suddenly, the person is diving to thirds, getting bored, and then doing jumps and T's. In some caves, you can easily hit a T before you hit thirds. It's easy to justify. I don't know if I agree with the expiration, but I totally understand it. Although it may stifle a diver's opportunity to grow, it DEFINITELY tries limiting people from getting complacent and pushing beyond training limits. There's no scuba police, but the agencies have to try to enforce what they think is best.