“Second, if you read the TOS, you are not guaranteed to have coverage everywhere you go.”
That is in the event there is no coverage, not with their complicity in shutting it down. That is theft, pure and simple.
“But since BART is a quasi-government agency, they are probably allowed by law.”
There is no such law.
You are assuming the carriers had anything to do with it - it doesn't sound like it.
Also, it is only illegal to jam cell phone service. It sounds like they just shut down the repeaters/antennas that they installed themselves to provide service to the riders. There is no law that says they are required to provide that access in their stations - they just can't jam signals that originate from oustide the stattion. So if the station cannot receive a signal under normal circumstances, I don't see how BART can be required to provide that service.