Well - is it a rule, or it's not a rule?
If it's a league rule, then it's not cheating (a strange rule, but not cheating).
If it's not a rule, then yes, it was inappropriate for the people running the scoreboard. But actually, it's not usually the scoreboard that keeps the official results, it is the home team recordkeeping. So the scoreboard doesn't really mean anything. After all, if you looked at game records that the home team should be keeping, it would have 0 for the points scored (because they would have to write down who score the points, pass, or run or field goal, etc, and those are all 0. And if you tried to just add scoring plays, you would mess up all the per-player stats).
Think of it this way - how often do you see officials in pro games make announcements for the game clock to be re-set to match the official time? All the time. They can do this because the scoreboard is not the official record-keeper.
So either way, I wouldn't get too bent out of shape over this.
ADDED: Interestingly, there is a recent huffpost article about the pointlessness of dishonest praise, and how kids don't really take pride when they know that the praise (or score, in this case) isn't deserved and that it undermines the credibility of the person or group giving the undeserved points (or praise). http://www.huffingtonpost.com/kyle/the-dangers-of-overcel...
If the link doesn't work, google "the dangers of overcelebrating our children"