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.