It's clearly got nothing to do with the safety (or otherwise) of the fixed staircase and everything to do with preventing the stampede of absolute muppets who try and race down the fixed staircase to effectively queue-jump.

I know the OP isn't in that category, but every time I've gone through T5 when I've been in one of the higher boarding groups there's always been a succession of clowns running down the stairs to try and get in front of people in the queue.