If a book is offering +120 on one team, and -130 on its opponent, it is the books job to get as balanced action as it can ---unless it is doing its own gambling. If it starts getting one-sided action, it needs to change the odds to attract action that will balance what it already took. The odds that the book publishes are public, so yes, it can give the same odds to all comers. The way to balance action is by changing odds for everybody, not discriminating against a few of their customers.