I would assume that during the regular season, the aggregated betting amounts are probably close to evenly split, even if the lines are not necessarily set for that purpose. After all, the house almost assuredly wins on the aggregate betting.
As for a game like this, the casinos are not assuming that much risk. Yes, they could lose money if the Pats win by 4 or more. But the casino also wins when the bettors make various side bets (score at half time, etc.), or make bets on their alma mater's college basketball game, or stick around for a few rounds of blackjack or roulette, or consume a few beverages at the bar while watching the game on the big screens. The money the casino would have to pay out is a relative blip in the ocean.