Sportsbooks will open lines intelligently, but they absolutely do move the line in response to market forces in an attempt to balance money on both sides, because when the money is balanced, they are guaranteed profit.
It's true that when you make a sports wager, the house is paying you out of their wallet. It's also true that they employ a lot of energy and expertise in order to open the betting at accurate odds. However, no corporate, end user facing sportsbook is themselves fading action on one side of the match intentionally. They aggressively try to balance money on both sides so they can guarantee a profit.