Punter: besides cancelling the bet, was your honesty for sending the email rewarded? a Bonus Play, or betpoints (if applicable), or at the very least by some goodwill built up with management? If not, this strikes me as a strategic mistake by management. While I doubt most of us expect a book to honor an obviously reversed line, particularly if the cancellation occurs before the game starts, if management doesn't give you something to recognize your outstanding ethics, then they're just begging for you, and by extention those that read this thread, NOT to alert them in the future.
If it were my call, I'd allow the bet of the first person to alert me to the reversed line to stand at the advantageous line (unless it's for a maxed amnt -- in which case i lower it, but still keep it). Otherwise, a book's just asking to be the last people in the know, which will cause them no end of angry clients and reputational harm. So, punter, did you really get nothing for such commendable honesty?