Really the default assumption should be that the employee desires to use his/her PTO and if they were unable to do so that situation would be a failure on the companies part (not having enough coverage, over promised a schedule, etc) and any PTO that failed to roll over should be paid out.

In my view that puts the incentives in the right places, firms that are unwilling to abide by those clear rules are likely trying to game the PTO system anyway and are committing a variety of fraud when they hire someone under those false terms.

Legal in California: You earn 2 vacation days per month, to a max of 24. Hired in January, never use PTO, by December you have 24 days banked. In January you fail to earn PTO because you hit the cap. Total balance at end of January: 24 days banked.

Illegal in California: You earn 2 vacation days per month, and they are "use it or lose it." Hired in January, by December you have 24 days banked and never use them. The next January, your existing PTO balance is set to 0 and you are not compensated for the unused days. Balance at end of January: 2 days banked.

The legal distinction is that your employer can decide not to give you PTO for any arbitrary reason (there is no legal requirement to give any at all), but they cannot take back PTO that has already been paid out.