Because its not important enough for me to care so if he doesn't do it just because he doesn't want to then so what. This isn't a court and we're not lawyers so why should he or anybody have to provide proof? I only do it in my posts not because I think I'm going to change the mind of the person I replied to but for outsiders looking in. It was the first thing I learned about debating is you are not debating to change the other person's mind, because you will never do that, you are debating for anyone that maybe on the fence and to sharpen your own points and strengthen your own case. In other words I provide proof and do my research because for me personally anything less would make me feel stupid and unprepared, if someone else feels differently then so be it.
If he wants to post without any proof why would you or anybody that disagrees with him be upset about that? It just causes those outside observers I mentioned to take his points less seriously so therefore makes your job easier. Unless you actually thought you might change his mind, in which case you are fighting an up hill battle. Arguing with Steel is like wrestling a pig in the mud, the pig likes it and doesn't want to stop so either you stop or you learn to like it yourself. I learned to like the mud long before I came here so for me it was just another play pen.