I don't see an argument for this to be a requirement. Before this implementation there were plenty of teams with no listed SF, for example. What's the point of requiring it now?
Think about this other situation... Let's say you have a SF that you'd really like to list as a PF, but he's the only SF on your roster. In order to make that player appear as you'd like, you have to change *someone else's* position. That doesn't make any sense.