I don't know about that...
If Manu had wrapped Allen up from behind: Good, clean PO foul.
If Manu had smacked the ball and caught Allen's arms: Good, clean PO foul.
Grabbing an airborne player's wrist is a recipe for injury. Yes, Allen exaggerated the impact and after-effect of the fall; however, that doesn't factor into whether the initial contact was "unnecessary." Remember Juwan Howard whacking Derek Anderson in '01? Separated shoulder...out for the POs because of one single "good hard foul" on an airborne--and therefore vulnerable--player. The special rules and special treatment of vulnerable players are in place to protect players from injury. Different story if Allen doesn't leave his feet; different story if Manu doesn't grab and pull Allen's wrist.
to see the rest of my "oversized" signature. "Leadership is diving after loose balls." -Larry Bird