The intent of the Fire Order is that the fly screen doors be removed & not reinstalled or replaced with something similar.
Depending upon how it is constructed & installed a fly screen door may:-
1. Reduce the unobstructed width of the doorway to the unit.
2. Act to reduce / obstruct the width of the escape route.
3. Have a locking system that prevents escape without a key (not a good idea in an emergency situation).