englishal, all valid points, however to identify all you suggest as a total outsider, not knowing the pilot, would be 'policing'. My thinking is more along the lines of those of us who have suspicions. It's all too easy to hide behind the, 'it's not my job' attitude. Luckily I have never had cause to consider taking action (other than checking out an admin. error on G-INFO re an aircraft that did carry public). This incident has made me question how I would deal with a situation where I did have reasonable suspicion. Hopefully I will never be tested as I have no wish to be put in the position and would find it extremely difficult. Equally if I chose not to take action and a similar event had to occur I would struggle to explain it away with the points you make.
Another way to think of it would be to consider how you would feel if it was someone close to you that was the passenger and you discovered that others had doubts about the legality of the pilot, deciding it wasn't their responsibility.
That said there are always going to be some who slip through the net.