I'd blame the user. Speculation: when you add an event you can choose whether you add it to the work or to the personal calendar and those people chose to add i

I'd blame the user. Speculation: when you add an event you can choose whether you add it to the work or to the personal calendar and those people chose to add it to the work calendar, which is public so others in Google can see when that person is busy, which in turn is useful to schedule meetings, etc