Nobody is saying they would blame their opponent for not telling them, in fact everybody on here is saying the opposite, they are just saying that when comparing telling your opponent vs. not telling your opponent, telling your opponent shows better sportsmanship.
Which act would you say is nicer? Giving to charity vs not giving to charity? Obviously giving to charity. While there is nothing wrong with not giving to charity, giving to charity is the more altruistic thing to do.
Here's a more pool related example which occurs every once in a while (I have even seen it happen among very good players). You are playing one pocket and your opponent forgets what pocket was his. Do you tell him? In no way are you required to tell him and you are being honest according to the rules either way, but to intentionally let your opponent shoot at the wrong pocket is certainly not the best way to show good sportsmanship.