The sport books actually pay a service to "set the line" -- its where the initial spread opens. The market then moves the line. Its the same thing with IPO pricing.
Offshores post the lines at low maximums and the market sets it from there as the maximums get larger.
Everyone in Vegas moves on air based on what the legitimate offshores do - i.e Pinnacle. The terrible companies that run the books in Vegas just piggyback what offshore already solved for.
You can watch it happen all day with any odds service. Pinnacle, Bookmaker and CRIS move and everyone moves right along with them.
The idea that everyone is trying to take equal money can be disproven just by watching the lines for one game... say UConn and Syracuse Saturday.
On 9/19 Pinnacle moved UConn to -5.5 at around 10am. William Hill opened UConn at -5.5 that afternoon.
At 5:37 Pinnacle moved UConn to -4.5 and at 5:43 to -4. William Hill at 5:45 moved UConn to... -4.
Now did William Hill take enough money in 8 minutes to move the line 1.5 points or did they just follow a book that actually takes risk from legitimate players? Are they really going to balance the plays following someone else's line moves?
WH follows the line bouncing around at Pinny for the next few days. On the 22nd at 4:46 Pinnacle moves the line to UConn -3 for the first time. I wonder what happened at William Hill... in less than an hour they hit -3 for the first time.
Two lessons here:
1. Nobody can possibly try and take equal money on both sides while
moving on air. If you follow the market regardless of what you've taken clearly you aren't trying to take half in each side.
2. Vegas doesn't set any lines. The market is already liquid and the lines have been hammered before any of the lame companies in Vegas even post a line.