As an over zelus student pilot I was sent off solo to practice insipient spins in a "long wing" worrier. In my enthusiasm I did what I know now (as an aerobatic instructor) to be an academic spin entry. The ship turned me inverted then the stability took over, gently flipped me back over and went spirel. I have since spun many "slab wing" cherokees and even with a trickle of power and different flap settings I've found that they just don't stay in. 1 rotation at most then spirel.
So in answer to your original question I think it because that don't stay stalled and the ensuing and possibly un diagnosed spiral dive might put you through Vne.
Having now done my fair share of dumm stuff in light A/C I also strongly agree with the chap/chapess who said don't spin anything unless the ship is cleared and you have a chute.