We have passcarded doors and then inside we have gates like many subway stations do that are timed only long enough for one person to pass through.
So I can hold the door open for someone on the way in—especially if they have their badge out— but there's nothing I can do about those giant plexi gates once inside. They have to swipe.
And since they’re entry and exit, there’s many methods to enter by triggering the exit side, from umbrellas to a small dog.
The specifications for those gates almost certainly include a requirement that they allow a sufficiently determined person through without breaking themselves, and probably sound an audible alert.
The gates at my work are enter/exit as well but we have to swipe out.
The social stigma, unpredictability and inconvenience of having to pay the fine is a big part of it (not having a ticket is just stressful). Another part is that you don't need to prevent freeriding, nor recoup all the lost ticket sales. You just need enough of a nudge to keep most people honest most of the time.
Obviously "not having too many unauthorized riders" is a very different objective from "only allowing authorized access and refusing all else".
Nuisance riders are: disturbing peace, damaging proprty, assaulting passengers or staff.
Police behaviour, not fares.
(And have social and justice systems which can effectively deal with individuals in need of help or discipline / isolation.)
They allow one authorized person to pass from one side through but sensors on both sides can easily detect if another person is trying to piggy back from the other side of the door. And there's no way over or around them. In higher security environments where unauthorized persons getting on the other side of the door is already an unacceptable risk they can also allow security personnel to trap someone in the door (rotate only 90 degrees).
And as far as usability and efficiency goes, they can allow traffic both ways at the same time (as long as both people are authorized).
Granted, they might not know the difference between one person and a handcart versus two people where one is in a wheelchair, but I doubt many would-be infiltrators would draw attention to themselves that way.