I find the fired employees defense of 'internal openness' to be disingenuous to point of being offensive.
Google employees are going to be using Google Calendar for work, which means that they almost certainly be using it for their private calendar events as well. It may be that other Google employees with access to their work calendars incidentally get access their private events as well. Even if they don't, the ability to get alerts about changes to the calendar mean that the appointments that incidentally get added as work events, but are immediately changed to private/personal events, end up not being private.
If those employees were stalking others at the company by having calendar event change alerts, that's exactly the behavior that Google should be firing people over. You are always counting on your employees to be ethical and responsible. Even if you can implement technical limits, and often you can't, there are always holes. The lack of technical restrictions on what they were doing isn't relevant, it's the behavior that is at issue.