It used to shock me too when I lived in Newcastle. An entire underground network built on trust! And as a smelly student I did regularly bunk the metro. I never had my tickets checked. I regret doing that now. The ticket prices were a pittance then anyway.
A friend of mine did get caught, and was held for 45mins, until he was issued with a fine. Not worth the effort in retrospect.
Mind you, the fine was only a fiver, so you could quite reasonably weigh the cost of a fine against never buying a ticket.
But as I'm older now, that's just plain wrong!
Mind you nicking blocks of Chedder from the blind-spot in the Presto supermarket in Byker was priceless.