The really baffling part is the fact that because of the way Matchbook calculates the fee charged, they actually create fewer fees via all the people who buy early on in an attempt to middle/scalp.
Say you start with this wager
Wager 1
Middler +100 $100 Over 49.5
Other person #1 +100 u49.5
If they don't move the line, our middler just sells their bet for a guaranteed profit in wager #2.
Middler u49.5 +135 85 to win 115 (profit $15)
Other person#2 o49.5 115 to win 85.
In this scenario, Matchbook only collects their 2% commission on $100 or profit people made if the game goes over, or $115 if it goes under. However if they offer the new line at 51.5 and lure some people into trying for the nearly free middle, they'll generate 2% fees on $200 of profit.