I don't know how it will work in Australia. Here leave accrues by law. Min leave here is 20 days but some companies gives more like 20-30 days. When you leave the job all accrued leave has to paid based on the last salary.
If it's a valuable option to you, you'll be more likely to seek employment with companies offering it. If you feel like you're losing value due to the lack of accrual, you won't. And if you're already at a company that makes the change? There are other jobs.
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.