No one expects them to have a crystal ball and no one expects them to be perfect, but that doesn't mean that the mistakes get wiped off the ledger. We are just a bunch of fans without inside knowledge, expertise or a professional coaching staff. All of us drink the FO Kool Aid to a degree, because we rely on what they say about a player as they are the professionals, the experts, the scouts. Most of the time we base our opinions on what the professionals say about a player. When they are wrong, they are responsible, not the fans. They are trained to know better than us. If you want to give them a pass, that is fine, but we all have right to give our opinion of their moves, and that correctness or accuracy of that opinion rests on the track record of the players and the team. It has nothing to do with prior opinions.
Edit: I could somewhat see your point, if the poster didn't also acknowledge that he/she was wrong too, but whether or not the poster acknowledges the mistaken opinion, it doesn't change whether the move was good or bad. The results speak for themselves and the FO makes the big bucks to be responsible for the moves.