Staff quite rightly think about conflict avoidance, hence some people getting let off. It's frustrating, but wait until the TOC does a proper sting with lots of their 'best' staff and perhaps BTP. People don't get away with anything then.
The plain clothed checks (not seen one in ages) are also very effective as you can get right up close to those who are quite happy to talk not having a ticket, fancying a 'free upgrade to first class' and doing other dodgy things. I think more checks should be done like this, and I know RPIs quite enjoy dress down day (their choice of clothing can be hilarious, like the few that liked to dress extremely chavvy!).
I was quite annoyed when one person was let off with a penalty fare, even after having admitted that he wouldn't have bought a ticket if not checked and hadn't attempted to buy one before travel. He was French (there's a reason for saying this!) and one of those that had the £20 in his hand, being very smug as if he knew the score.
I asked the RPI after (away from the person, I might add) why he'd done that as it was a clear intent to evade the fare, and he suggested he was likely a tourist. Hmm, a tourist that knew to have £20 ready and was constantly looking behind him before the RPI boarded at Finsbury Park right in front of him! Are we normally so welcoming to tourists? Maybe I need to practice my GSCE French from now on, or at least speaking English with an accent.