Reasonable bonuses will not bankrupt an even marginally competent book. Couple points about this:
1. Bankroll. In theory, the book's bankroll should be much, much larger than any players. A large bankroll with even a slight edge will take out a small bankroll quite easily. Yes, many books are undercapitalized, but that is another problem entirely.
2.Money management is basically absent for many players. For instance, Joe Citizen deposits $1000 and gets a 20% bonus, leaving his account at $1200. Do you think he is betting $10-25 per game as his bankroll dictates (1-2% of BR)? Hell no. Joe Citizen is betting $100+ per game, because he has no idea of money management. This greatly increases his chances of losing it all on a bad streak.
3. Gambler psychology. Players push when they are up. Players chase when they are down. Players bet shit games just for the action. They play parlays, teasers, etc without an edge. They play more on Moday night Football, championship games, and TV games. Just the fact that they have money in an account and have lines in front of them, they are going to bet. Again, a winning proposition for the book. Joe Citizen isn't going to go through the hassle of withdrawing from one book, depositing at another book, withdrawing to his bank account, posting up again, etc. He's going to just leave it in there, and now has to face the temptation to bet every game. Average players have no discipline, and just getting their money into the book almost guarantees they will lose it. If Joe Citizen deposited $1000 and now is down to $500, will he withdraw? Hell no, he's going to play until he gets back to even or loses it all. When he gets back to even, he's going to gain confidence and keep pressing until he loses it anyway. This facet of the psychology of gambling is well known. I've seen this send my friends into debt countless times. So, most players are never going to withdraw dollar one, meaning the bonus is negligible to the bottom line.
4. Bonuses are a form of advertising, a natural cost of running a business. Instead of running ads, sending flyers, etc, their advertising budget is spent on bonuses. Same basic cost of any business, just a different method.
Bonuses are just the hook and bait to attract "square action". Obviously these points don't apply to the rare "sharp" players out there.