While I understand the reason for this, it raises some potential problems in my opinion.
It doesn't seem fair to require a school to hold the scholarship for a player (or multiple players) who wants to test the waters in the draft so they can return if not drafted
On the other hand, if you say undrafted players can go anywhere and play immediately, I can see most players who want to transfer declaring for the draft so they won't have to sit a year
Seems like it could get messy