I certainly don't have an answer to that one, but I know that it drives me crazy. It's not just one tube site that does it, but pretty much all of them that I have found. I can only think that they do it in order to drive the search results to that particular video. For example if one tube site favors a lot of twinks, and they put the hairy guy video on it, then they might label it with "twink", "teen", "smooth" in order for that video to show up in the results. And hopefully you would watch it and then go to the site that it is from.
I think it sucks that people who load the videos do this, but I don't think that there is any way around it. It's just something we have to deal with.