Why is the Green Line free after Kenmore and Symphony

Just out of curiousity and not that I'm complaining, why is it free to ride the Green Line on the surface levels for the B,C,D,E lines? Is it like that in other cities as well?

There's no fare collection outbound so passengers can use any door to exit the cars, thus speeding up service (slightly). Fares *are* collected in the inbound direction. If you're riding round trip entirely west of the subway stations, you do get a break. But since most riders are headed to or from downtown points, most riders pay a fare in each direction.

Well, I decided to reopen this thread, since there was no other thread found. Although we already know why there's no charge at the surface, since it's going away from the city.