This is a bad versus bad Big Sky Conference matchup. So backing the home underdog makes sense especially given the situation and style of play.
Northern Arizona is 1-6. The Lumberjacks play slow, are weak inside and can't shoot from the outside. Not exactly a good combination. They average just 58.6 points.
So how bad is Idaho to be a home 'dog to this opponent? 
The Vandals are 0-5. But all of their games have been on the road. This is their first home game and it comes on New Year's Eve. That's certainly not a fun travel date for Northern Arizona. 
Idaho is the better defensive team. The Vandals also are a much better 3-point shooting team ranking 104th in 3-point percentage at 35.8 percent. The Lumberjacks are 319th in 3-point shooting percentage hitting 26.5 percent from beyond the arc. Idaho has more size than Northern Arizona. 
I realize nobody cares about this game, including the oddsmaker. But value is value and I don't see why Idaho should be an underdog here.