I think fining them is harsh and punitive, but at the end of the day McCarthy has chosen not to be competitive, and has completely wasted a matchday for the fans. If United win the title by 3 points or less Chelsea would have reasonable grounds to appeal.
If United put out a "threadbare" line up they would still be looking to win (ie being competitive). McCarthy is effectively choosing not to compete, which like I said with Burnley getting a draw on the same day at Arsenal, shows that it was daft.
Of course there isn't fair competition between all teams, are you suggesting we should do everything we can so no teams are better than others?