The underlying part here is that these employees were using the notion of "unions" to organize _politically_.
...and their willingness to act against fellow employees and leak company documents demonstrates just how dirty they would have become had they been allowed to continue.
Personally, I don't think they cared at all about worker rights beyond their desire to advocate their politics.
Why were they using company resources for personal meetings?
Probably because it represents time that are unavailable for work and, in many cases, because their are people at work (including their boss) entitled to know the reason, and who probably established that as the workflow for sharing the information.
So that coworkers don't schedule you for meetings during your doctor appointments.
Remember, the so-called stalking was looking at information that the owner chose to share with the company and didn't mark the "private" flag on (which is available as a default to apply to all meetings).
Somewhere along the line, people started scheduling me for events in conflict with other events. Why? My online visible calender was free and clear into eternity so obviously I have nothing going on at 2pm. Explaining not being able to be in three places at once got old fast.
So now I have an eye toward using the calendar "defensively". Here's when I usually come in, here's when I usually clock out, here's when I usually take lunch, here's that weekly meeting I have to attend for someone else's giggles, here's when I'm helping Jimmy with this thing, here's when Samantha is helping me with the other thing. You want a meeting? Now the calendar begins a negotiation.
I may or may not like it, but the tipping point has come and gone for me.
Signing up for email alerts about somebody else's calendar is definitely weird. But the information you'd get is the same as someone would get if they checked your calendar manually, which is an expected and normal part of Google's culture.