The one night per year for the teacher to meet parents, and they can't show up without "getting paid for it?" Please. Teachers work hard--but so do most folks. There are few of us that do good jobs at anything that don't "give extra." Cry me a river.
Again, I don't know the specifics of the particular situation, however...
Most teachers are already under paid AND they give up many, many extra hours throughout the year (and that doesn't even count the countless hours spent thinking and mulling over various issues in the classroom) beyond the time they are paid for. Few professions give up more extra time voluntarily. Why? Because most teachers are in their job for idealistic reasons. However, sometimes parents and/or administration can take advantage of this willingness to go the extra mile, or, at least, begin to expect it as a given.
I don't know if that was the case here, but to portray it as simplistic as you do is unreasonable. As in any job situation where the clientèle and/or bosses begin to take advantage of the worker's normal willingness to sacrifice, sometimes the worker says "Enough!" and refrains from doing the extras they would normally do.
Given the standard behavior of teacher's unions (particularly ones in more populous areas), I doubt there's much more here than meets the eye. They were told not to show up due to a contract dispute, and they fell in lock step with their union bosses.
Possible. Just as likely there is more to it as well. I know from having seen it from the inside. Can't really tell from the slanted news report. From what little I have been able to find, it looks like the admin is asking them to take what would work out to be a 50% reduction in wages. That right there tells me there is probably something else going on. No way to tell since it seems only one side of the story is getting told.