These days not only do I tap in everywhere, I'm so much of a goody two shoes that if I forget to tap out I log in to the nexus site to complete the journey.
But 15+ years ago, back in the days of the yellow paper tickets, I never bought a ticket because - and this is the absolute truth - I was checked once in 3 months, and realised it was cheaper to pay the fine than buy tickets every day. I then didn't get checked for about 9 months, and saved (well, drank) an absolute fortune.
So yeah, if you're not going through stations with barriers, it wouldn't surprise me at all if people are free riding.