You're right if the bettor is a small player relative to the size of the market. But in the real world of MMA betting, total take is quite low even for big fights. I'll often move a line by 20 cents just by risking $15k on it. So the reality for a really successful player is that he either bets the line himself or he offers a play to his subscribers---either way the line moves and the value is eliminated.
I imagine touts in the big three sports can have their cake and eat it too, but it's difficult in a niche sport.