"Though I guess people would just tap in, tap out immediately but don't exit the turnstiles, then just get on a train and exit through the emergency exit at whatever station you want for free."
Sure, or just jump the turnstiles and exist through the emergency exit on the other end, which I already see people do frequently.
Sure, but jumping the turnstiles is clearly ipso facto breaking the rules. Re-tapping where you entered but not exiting immediately could be explained away. I bet far more people would cheat if they only had to tap to exit and not exit, versus jumping turnstiles.
Sure, or just jump the turnstiles and exist through the emergency exit on the other end, which I already see people do frequently.