Originally Posted by
Scifience
It's a technical issue with the way the fare system is designed—while Beijing is currently a flat-rate system, there are plans to migrate to a distance-based fare scheme like that used in Guangzhou and Shanghai. In the case of a distance-based system, whether or not your token is valid for exit depends on where you got on, which is coded on the token at the time of purchase. It's not a poor implementation; many other countries' systems (all of Japan, for instance) work the same way.
That still sounds poorly implemented. In a distance-based system, a fare card is checked at exit and the fare deducted. If it's checked at the entrance, the only thing that can be checked for is whether the card has a sufficient balance to either the lowest-fare possible or the highest fare possible (i.e., you have enough to get on, or you have enough to get off anywhere). It can't check for your specific destination.
Given that, the entrance readers should be set to recognize that the fare card has a sufficient balance to exit at any station.
Moreover, if there is a change to a distance-fare system, the system will need to be capable of accepting cards bought wherever because patrons will reload them as necessary.