Billions of dollars, and most of it square money is entering the market during this time, that is not a factor during the regular season. Lots of parlay risk, whether OTB, teasers, or cards. Futures, and contest payouts to worry about which again is not a factor during the regular season.
I live in Vegas, and the books on a regular day open around 8/9am. All the books in town are opening @ 6 in anticipation of the floods of tourists. Also Vegas is such a small part of the pie now. Offshore are the ones taking serious risk, whether in Costa Rica, Europe, or China.