A few thoughts:

1) if this was happening, how would you know? if it was happening, the effect could be to bankrupt one or more of the syndicates to the benefit of one or more of the other syndicates. IOW, the bigger sharks would eat up the little sharks, but the net effect to you could be the same, the big shark now has more money with which to move the market.

2) it wouldn't be too hard for them to learn to avoid each other, they probably don't even have to discuss it but why couldn't/wouldn't they?

3) at big tracks, they may just not have enough money to bet, or may just prefer not to bet so much money to move the odds that much ("You can sheer a sheep many times, but skin him only once!"). HANA says the average mutuel pool at, for instance, Aqueduct, was $225k/race, so you could bet a decent amount without moving the odds appreciably.

4) don't underestimate the value of rebates - if these guys can get a 5% rebate they're *thrilled* to make 0EV bets, they don't care where the $ comes from as long as it comes.

5) don't underestimate the amount of money that they're funneling into the exotic pools, which is much tougher to see/feel/detect.