I think that when cell-jammers first became available, the FCC said that they were illegal regardless of whether private citizens used them or municipalities. The FCC is the regulatory body that has the last word on radio technology, and since it is a federal agency the whole area is under federal authority; it isn't an area states and cities can enter into.
This has nothing to do with cell-jammers. BART normally offers cell phone service underground as signal usually can't travel there. They just flipped the switch.
