Horsemen and track management in California have so far refused to allow it.
At the last CHRB committee meeting I attended owners and trainers gave presentations and cited the commission percentage Betfair and other exchanges typically charge players as being too low. (What a surprise: They're demanding higher takeout.) They also cited the ability to bet against horses on an exchange as a concern.
Track management voiced their own objections.
I came away from the meeting with a strong suspicion that both Magna/XPressBet and CDI/Twinspires are quietly working on their own exchange wagering software platforms - and that if either or both had exchange software that was ready for the outside world - no easy task if you really look at everything that Betfair's platform does... both horsemen and track management would consider doing a 180 and endorse exchange wagering. (That's my take on the situation in California for what it's worth.)
That said, I wouldn't be at all surprised to see New Jersey be the first state to go live with exchange wagering on horses in the US.
Neither Monmouth nor the Big M are owned by Magna or CDI and neither have pipe dreams about developing an exchange wagering software platform and marketing it to the rest of the industry.
Also, unlike the tracks in California, both Monmouth and the Big M - as well as the Governor's Office- have demonstrated they are not afraid to stand up to the horsemen in NJ when the situation calls for it.
Based on all of that - the tracks in NJ appear to have fewer objections to partnering up with Betfair to operate an exchange. (For what it's worth that's my take on the situation in NJ.)