The turnstile is actually a great analogy for DRM, including all of its flaws. And the turnstile isn't what allows subway service to exist.
A turnstile isn't very hard to jump over. If the turnstile is the only enforcement mechanism, anyone who wants a free subway ride just has to vault over (or perhaps also glance around to see if there are cops nearby, google whether there are monitored cameras, or pull a hoodie over their face).
There's an alternative, though: most places in Europe, there are no turnstiles, and nothing preventing you from getting on public transit without paying. It's a "trust but verify" system: you are required to validate your ticket before you get on the train, and occasionally someone comes around the train to check if you have a valid ticket, and issues a fine if you don't have one.
With this method, the users aren't inconvenienced, enforcement costs are reduced, and everyone's happy. With the turnstile, you get massive queues at busy stations during rush hour, and by making this one mechanical device your sole point of enforcement, you've actually made it more likely that people who don't want to pay will cheat the system.