Since 2003 they are .524 against the spread. Higher than average, and still squarely in the middle of the distribution, behind many other teams.
And yes, the "bias" you speak of would have to affect all bets in order to move the market. Your theory rests on the assumption that professional bettors hate money and would rather lose on average than bet on BYU.
If you made a living betting spreads, and found a random CBer's secret (BYU lines are biased) to be true, you'd bet on BYU in every single game. It's clear you don't know how capitalist markets work, but bettors would keep being BYU in each game until the line wasn't biased, because it's a winning bet up until that point.