I suspect sharp bettors don't trust USC's coaching or freshman QB. The advantage to playing Slovis this week (as opposed to Stanford), is BYU has a full game's worth of tape against a good defense that they can use to analyze Slovis and get the players ready for his tendencies.
I think USC wins by 10+, so I am surprised the line hasn't moved, but sharp bettors see bad coaching, on the road, and a true freshman QB (and USC struggled with a better true freshman last year), and they say 4's about right. Particularly if they expect BYU to run the ball (which I do) and try to keep the score low.
The total is at 56 (give or take), so the Vegas predicted score is 30-26. That seems high to me. I'd take USC and the under, but I only talk about betting, I don't actually bet.