I am simply saying it is poor sportsmanship to knowingly allow your opponent to foul or for you to not admit to a foul just because your opponent didn't see it.
I had 3 things happen last night in my APA match that showed good sportsmanship.
First, my opponent got down to shoot in a very easy 8 and both myself and my score keep told him to mark the pocket. It was an easy tap in for him, so he just quickly shot it in. I personally did not care, it was obvious. I went and spoke with my team captains to make sure they are OK with that as I have been reprimanded by another team captain for not enforcing the rule. So I not only try to prevent him for losing, but I then allowed him to win and just asked that he mark in the future.
Second, my opponent took a very close hit. One I thought was good, but he said it was not and I was down to my last ball and the 8 ball so he basically gave me the game.
Third, I tried to play a very soft safe using the 8 that would have in all probability given me the win but barely missed the rail (about a millimeter off) on the opposite side of the table from where him and his team were sitting. They all had no clue I missed it and were surprised when I picked up the cue ball and handed it to him. Again, that cost me the game and the match as it was Hill-Hill.
Those 3 examples are players playing with integrity and good sportsmanship regardless of situation. Those that say it depends on the circumstance as to how they will play it then I will simply say that in my opinion, you are a coward and a person that is willing to compromise your integrity if it suits you and I am sure that doesn't end when you pack up your sticks.