I always thought these FN keys were hardwired in and couldn't be changed, even with a firmware change.
I remember noticing one case of that in a keyboard described here, for its Turbo key.
I would not have been inclined to think this about Fn keys in general, though, since it's
much cheaper to design the circuit so that this key is also part of the matrix. In general, though, they would still
never allow the position of the Fn key to be changed... because then you could lose track of which key you had to press in order to change it back. Don't want to generate those extra Customer Support calls!
So the reason isn't a technical one - they're just dumbing down the product in self-defense.