London Banter - fare dodgers

Wolfgang Schwanke wrote:
Hello again,

during my last London trip I witnessed a group of police and LU staff
"guarding" the exits of the station. They caught one man just coming
out of the station when I was there, he seemed rather upset (I didn't
see how he got through the fare gates). Also inside the station I saw a
policeman taking down the ID of some passenger. My interpretation was
that they were on hunt for fare dodgers.

The part I don't understand is why they do this in the first place. I
thought the LU system is rather "fraud safe". You can't get in or out
of the system without some form of valid ticket (except by climbing
over the gates or taking a detour to a DLR station), so hunting down
fare dodgers should be pointless. What am I missing?

The event was in Archway BTW.

The culprit could have been travelling on a child ticket.

Regards