Is this legal?
If legality is the primary concern, this should be asked in law stack exchange.
However, if the question is more about the practice, opening up the calendar to other employees and managers is not unheard of and since you already understand that it's only for the company use, as long as it is visible to only company employees, I do not see a concern there.
Your manager (and manager's manager, and so forth) is responsible for managing your time, and having a look at your calendar is one of the easiest ways to do some of that work. If anyone is not using the calendar booking to steal time (useless meetings / unnecessary meeting bookings etc), there's nothing wrong in having an open calendar.
Personal example: In our team, we have calendars visible to whole team and manager (+1) and Business Unit head (+2) level.