It's incredibly common all over Europe, not just Switzerland. Not only the metros but the trams and even buses often rely on this system where there's no turnstile or barrier, you just walk in.

Not sure it's about being a high trust society or not, there's frequent inspections where they block the doors, and you get a hefty fine if you're caught without a valid ticket. I certainly wouldn't call Prague or Rome or Dublin high trust societies on par with a Swiss city.