A local ABC affiliate describes the situation this way: "BART is rolling out a new plan that will allow it to ban passengers who don't behave."
But the RT blog describes it this way: "New law will ban protesters from riding mass transit in California... that power could be used to prevent political protesters from getting to demonstrations or essentially going anywhere."
The law is here and states, among other things, that possession of any illegal substance, "lewd" behavior, "unruly" behavior, or pretty much anything a transit cop decides is undesirable—they get a lot of discretion—can lead to a year-long prohibition from public transit. (It seems one must have gotten three infractions in 90 days.)
This measure was put in place, in part, because of repeated protests against BART in San Francisco after a BART cop shot and killed a mentally ill homeless man. (A few years prior, a BART officer shot and killed an unarmed 22 year-old.) BART, of course, defends the law as an attempt to "create a safer, cleaner environment for BART riders and employees."
But even the ABC report recognizes that "AB 716 won't only target violent behavior. It can be applied to protestors who have been arrested during free-speech movements."