It's also yet another way to ensure people who know they can just walk into another job (or that they won't get fired/laid off/never promoted) take loads of vacation, while people who _really_ need to be sure they stay employed or who might be having a harder time at work are always afraid to ask for it.
Buddy of mine worked for a company with unlimited PTO until recently. He would take 2, 3 months off because, as he put it, "I had them over a barrel" (he is extremely good at what he does). His colleagues, for the most part, did not.
In truth, though, the sole reason companies use unlimited vacation policy is that, in the US at least, it means unused vacation time does not accrue as a liability on the balance sheet, and companies aren't required to pay out for it if you leave.
I also have unlimited PTO and I definitely prefer it. No more worrying about hours, rollover limits, accumulation rate, how far I can go negative, etc. No more wading through vacation schedules wondering if the office is open on New Years Eve.
Same thing with free lunches - yeah, of course it keeps people in the office and possibly causes them to subconsciously work longer. But also...I get a free freaking lunch and I don't have to worry about it.
I wouldn't take unlimited vacation as a sign of anything other than "we expect you to be reasonable and we don't want to micro-manage everything." Although if my experience had been different, I might think different.