Why are you positioning this like it's the employee's fault? I've had a gig with "unlimited" vacation time where my boss was very quietly stewing that he thought I was taking too much time. Never said anything to me until two weeks before I left.

It is in all cases management's job to set these expectations, and their fault when they're not understood on both ends of things. That's why they're management. (And why "unlimited" policies are stupid.)