I doubt that's it at all, just a simplicity thing. "If it's in the game it gets a tag" is easy to understand and there's very little grey area about whether or not something makes the cut, assuming you have your tags defined well, at least. If games are only tagged once the reach a certain threshold of content, you might be missing out on one of your favorite games just because you didn't find it when you searched for your favorite tag. There's no right or wrong way to do tags, just different ways.