I don't think it is a reasonable restriction at all. Besides the most devoted fans, I don't believe many families would purchase multiple licenses to a lot of games just to be able to play them simultaneously. These kinds of terms don't really increase revenue but they unnecessarily damage the experience at the same time. This is exactly the sort of stuff that makes pirating more convenient. As I've mentioned in another comment, the Mac and iOS App Stores don't really have these limitations.