I probably didn't do a good job of explaining it -- it looks confusing even to me this morning. Here is an article that may explain why they were flagged. What Idaho did would be considered a "hideout" play and is illegal.
http://www.referee.com/more/Samples/...rickplays.html
“Hideout” plays, in which a substitute or an apparently replaced player takes a position near the sideline to deceive opponents, were banned in 1942 by the NFHS. The NCAA followed suit in 1953 (NFHS 9-6-4c; NCAA 9-4-2b).
In a 2004 playoff game, Wasson (Colo.) High School huddled with 10 players after a kickoff return. Just before the huddle broke, the 11th player came onto the field. The player he apparently replaced simulated running off the field but stopped near the sideline. That player ran downfield at the snap and was not noticed or covered by the defense. The pass was underthrown and dropped by the intended receiver, thus avoiding greater embarrassment to the officials who did not recognize the foul.