Neil Williams wrote:

Or just enforce that touching in and out
is mandatory regardless of what's on your card.

Then explain how you'd handle the following situations:

1). I sometimes visit my parents in Epsom, for which I get an extension to
my existing season ticket. I will not be able to touch out at Epsom so how
do I touch out. (And even if I can get special dispensation to enter through
the barriers without touching in, I may well not be on an Epsom train for
all of the journey, but instead changing at Sutton.)

2). I come home in the peak hours to a station where all passengers have to
squeeze through a narrow hallway where there will only be a couple of
readers at the side. It is completely unrealistic to expect everyone in peak
hours to stop to touch out on such limited facilities without serious
problems. There will be tensions with people who are slow, everyone will
have just spent four minutes literally ram packed into poorly laid out
carriages where tempers do get frayed (and it's only the sardine effect that
stops people being able to swing a punch) and there will be crowds stuck on
the staircase up from the platform. I can seriously envisage some people in
a hurry taking a risk and walking across the mainline tracks in the dark to
use readers on a quiet platform which has an exit directly onto the street.