That doesn't make sense at all. You can bet the over or the under, so why would Vegas want everyone to bet the over? Then you'd know to always bet the under, duh!
It'd be more accurate to say they create lines where the side that they think will have the most action is the one Vegas thinks won't win -- over OR under. If the line is 7.5 games and most people have money on the under, then you know Vegas believes it'll be over. Likewise, if it's 7.5 games and more people have it on the over, Vegas thinks it'll be under. So, they are trying to sucker you, but it doesn't mean it'll be over or under necessarily.
However, the lines change based on amount of bets just so they can hedge their risk. If 85% of people are taking the over on 7.5, they're going to increase the number to get more action on the under so they aren't taking on so much risk if they do win 8 or more. This is usually more applicable for single game bets, though. That's why you'll see some shitty Patriots-Brown game climb from +10.5 to +14.5 because no one believes the Brown can possibly win.