It's been accounting rules for a very long time. It's pretty much Accounting 101. If you accrue liabilities that you're obligated to provide (either in the form of employee time off or pay-out upon departure), that's a textbook liability.

Which is why employers almost always cap in some form or another; the exact mechanism is partially determined by state law.

I worked for a company once that, during a bad spell, eliminated an accrual cap to encourage people to bank vacation. The result was that a not small number of people who weren't really into taking vacation just let their balances balloon. The company eventually forced people to work down their balances.