Because it is. I told my new boss the old earned vacation structure and that at a minimum I plan on staying on it with a little bit of flex. He said 'okay' and we moved on. I have a screen grab of my accrued annual vacation from end of 2019 so if they challenge me on it, I have it to push back on. We'll see how it plays out long-term...Cause of course they are. My wife's boss's company is going after an employee for taking too much PTO, even though they have an unlimited PTO policy.
Methinks it's just an open ended way to make people take less. One of those, when its given you feel entitled to take it all, but when you must ask for it you'll do so less frequently.