First, let me preface this with a few statements:

1) I work in the wireless industry.
2) I AM NOT a lawyer, so do not construe this as legal advice. Seek professional legal council if needed.
3) Not all of the facts of this issue are known to the public.

Denial of service to wireless communications has been deemed denial of access to 911 emergency services. If BART had actually dropped the signal from the cell sites themselves, there would have been an issue.

Since BART owns a vendor neutral (carries all cell companies) DAS (Distributed Antenna System) they can choose to do with BART property as they wish. They have not disrupted access to the carrier designed and owned (and FCC regulated) cell site.

Regulated cell sites CAN be taken off the air in emergencies either by court order, or field expedient means (jamming).

Jamming of an FCC regulated signal (i.e. ANY signal) by a non-government entity is a big time NO-NO. Just ask all the casino's in Las Vegas, and any prison system. Jamming is not controlled by hard boundaries, so it can have unintended consequences. You can build a Fairaday cage, but good luck getting people into the casino. Kinda defeats the purpose.

As far as prison systems and jamming, the rulings have been that the prison system does not have an emergency 24/7 so jamming cannot be in place 24/7. In other words: Prison, do your job and keep the illegal cell phones out of the prison.

While nobody would choose to live next door to a prison, I am sure those that do would not want their cell phones jammed due to bleed over from the prison.

Enough said, I am not going to get technical or legalistic. Any questions?