This says it well:

"...it couldn't be said that it was an "unusually unethical" course of action.
But dude, you are trading your personal honor and integrity for just a few FF miles..."

If the airline happens to make a mistake that falls in your favor, that is one thing. It is not your job to clean up records when it is contrary to your interests, any more than you should show up at the police station and volunteer to pay a fine because you were speeding earlier in the day. On the other hand, if you are deliberately misrepresenting the facts to beat the airline out of miles, then that is another matter entirely and it is clearly unethical. The title of the post is "Cheating for FF miles". It takes some seriously tortured logic to say that cheating is not unethical or that it is somehow okay to do that.