Getting milage credit for a bonus award, where you know for certain you should not get credit and are only able to do so because of an administrative mixup is unethical.
Would it be a common course of action for most people? Sure, as evidenced by this thread and the other "ethics" thread. So 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. Selling out is the norm these days but, sheesh, hold out for a million bucks or a guest slot on the Tonight show or something.