Steel wheels flex. Fiat avoid noisy or lot trims by attaching them with the wheel bolts. Other manufacturers allow the trim to flex, with a spring ring inside which also attaches it to the wheel.
Different wheels have different rim profiles and aftermarket trims cannot fit all properly, which is why so many are in hedges. If yours are squeaking, without cable ties they will soon go quiet, as they go awol. With cable ties, you can't get them off if you get a puncture, especially in the dark and raining.