The regional subway system is clashing with protestors after its transit cops recently shot and killed and unarmed black passenger. Its directors are so terrified of the paint thing —- WHERE WILL IT STOP?? NEON YELLOW FACE GRAFFITI? — that audience members at their board meetings are now subject to frisking, can't clap or boo or bring signs or banners into meetings.
Since BART is indeed a government entity, there's little chance any of these rules will hold up to court scrutiny, since they blatantly violate not only the First Amendment to the U.S. Constitution but probably also the "non-disruptive recording" provision of California's Open Meeting Act. But they might just live until the worst of the shooting controversy blows over, which is likely what the BART board is counting on.
In the meantime, the idea of a "Twitter ban" may well spread to private organizations not so encumbered by the Constitution. Shareholders meetings, arts performances, restaurants, sports games — Twitter banning could well go viral. Come up with the dominant "no Twitter allowed" icon and maybe you can make a buck off the trend.