Some viewers hate Casey’s revelation scene because they think it robs them of Sarah’s decision to go with Chuck even before finding out the truth about the mole. Now, the problem with this view is that, if Casey doesn’t tell Sarah the truth about what happened at the train tracks, she would never find out because Chuck would keep his mouth shut for Casey’s sake, even though it means losing her.
Obviously, Sarah needs to find out the truth but only after she has already made her decision to go with Chuck (not too long after, however, since it would dilute the tension). The timing of Casey’s revelation scene is thus perfect because that’s exactly what happens—Sarah chooses Chuck even though she still thinks he killed the mole, and now she can find out the truth about what really happened.
For those of us who have watched or read Pride and Prejudice, this scene is equivalent to the one when Lizzie finds out through her sister Lydia who really saved their family honor. That person—whom Lizzie previously rejected based on prejudice just as Sarah rejected Chuck based on prejudice at the beginning of American Hero—also would never tell Lizzie the truth, even though it means losing her. And Lizzie loves him even more because of it. Same with Sarah and Chuck.