Mixed feelings. Understandably you're not supposed
Which they were choosing to do, but I believe this is also to get/create (more) attention for their cause. It's the tactic of shutting off the reception that I disagree with. Seems very totalitarian. Obviously it would not only affect the protestors but also people who are uninvolved in the demonstration.
No mixed feelings at all this is very scary that the government thinks it can get away with this and that it's citizens think it's ok. Now that this has been done once who's to say when and how they can choose to do it again? Are you ok with this regardless of party? Or only if your party is the one disrupting a private companies services?
There are parental controls for all the major carriers, limiting text limiting the numbers and when one can call etc, that is very different from a government agency deciding they are going to stop my service just in case there is a riot. Funny thing is The Watts riots and the Rodney King riots didn't need cell phones
On one hand if BART were a privately owned entity protesters would never be allowed on private property to do this but on the other hand if it were privately owned this wouldn't be an issue in the first place.
I think you neglected to mention the impetus for the demonstration. Apparently, BART officers have detained AND THEN shot civilians in the past. A man was killed a little over a month ago, witnesses say excessive force was used.
So, not exactly a march against austerity measures or anything.