To use a well-worn quote from Col. Sherman T. Potter .ret - "Horse Hockey!!"
WGI, from my understanding, only permits specific medical companies to operate within their confines so it's unlikely to be a "bill cramming" situation with a disreputable service/company.
Unless the track rental didn't explicitly state the rental fee included EMS services for any and all incidents (IOW, it was just a "retainer" to the EMS crew to meet event/sanction insurance requirements) there could be grounds for the EMS company to bill individual drivers.
At that point you get into what was stated in the event supplementals and whether entrants were fully aware of what their risks (financial & health) were.