I don't think we can say that not walking is cheating since there is no law stating that if a batsman believes he has hit the ball he must walk.
What we have here is a morality issue and we all live our lives very differently. What is black and white for one person is a grey area for another and if there is no specific law governing the act, it can't be labelled as cheating.
Personally, I'm a walker. Always have been and always will be. If I know 100% (and as others have stated you can't always be but if I'm certain) that I hit it, gloved it.... whatever and the umpire doesn't give it, I'm still walking because I know that I am out.
McManus must have known what he was doing but carried it through, he had a chance to call the bat back. For me it is not exactly the same as someone [knowingly] snicking one and not walking. Whilst I feel that both are immoral acts, one is cheating and the other is just poor sportsmanship.