It sounds like Lt. Millon is a good guy and most of his men are as well. The fact that there is a law prohibiting open carry on public transportation is, in my opinion, one of the worst laws on the books. Let's face it, buses are large and can be used as a weapon itself quite easily. Not to mention that a crowded bus is an ideal target for a BG looking to get something for nothing. The fact remains though, that even though RTD doesn't enforce the law, it doesn't mean, as you have found, that you can't be stopped by local LE. If RTD is against the law and believes that people have a right to carry on the buses, then they need to work and fight to have the law repealed. They obviously believe in the right to defend oneself and they agree with carry, but as you stated, I doubt there are people willing to be a test case for this situation.
Maybe with the repeal of the Denver OC ban the bus exclusion will be lifted as well. I don't know, but I would like to see it happen as buses are a great way to travel around Denver without having to waste gas and get stuck in traffic.