It's not exactly a scam, but it is a psychological trick of sorts. Unlimited sounds nice, but, of course, any employee would be fired if they started taking unlimited vacation.

One thing that's rarely mentioned is that for employees who accrue fixed vacation time, the monetary value of this time tends to be legally required in most US states to be paid out upon the employee's termination. I'd guess that with unlimited vacation time, $0 is paid out.