They don't always end up with 50-50 on each game. They move the lines and odds to try and even it up. They can't lose, if it ends up exactly even, but it rarely ends up exactly even, and my point is they likely win more of the uneven games than not, since too many bettors bet with their hearts not their minds.
Also, moving the line allows for a chance they lose both bets. Say the line opens at 3 1/2 and moves to 4 1/2. They could lose, if the game ends up at 4, if more people took the fave at 3.5 and the underdog at 4.5.