They Should Be Faster

Just a quick question: Why aren't Boston cabs equipped with Fast Lane transponders? I know the drivers are trying to make money, and a longer ride equals a higher tab, but considering how often they go through the toll booths, and considering the passenger isn't paying the toll (at least not directly), wouldn't it make sense for the cabs to have a Fast Lane transponder so they can zip right through and don't have to sit in traffic?