It's a scam because it promises something in theory that doesn't materialise in reality. Usually to the employees detriment

It is a scam in that the name is intended to mislead. "Unlimited PTO" is not actually unlimited. If it were called "un accrued pto" it would be more accurate.

Because the only reason it exists is to be able to circumvent pto laws: you meet the legal minimums because it’s “unlimited”, but you don’t have to pay out the accrued vacation time when they leave, despite logically the accrued vacation time should be their entire time working for you, as all the time could have actually been paid time off.