Can anyone explain in two sentences why, if someone has a NR return ticket
from London-XXXXXX-London, which is valid for any route, they are not
allowed to leave at Farringdon?

Why is there discrimination on people leaving the network at a station
which is not a terminus. I don't understand it. Why is it not to any NR
station within Zone 1.

It doesn't affect me particularly, but I just don't get the logic in it.