ok, they did at least try to reunite with the herd. Seems really odd that the herd rejected it. Also surprised that momma bison didn't nail the tourists if they were that close to the calf. Maybe she had already decided to abandon it.
They drove it around in the car - it probably had scents on it that they wouldn't tolerate.
I still don't understand why they couldn't have found a rescue for it when the herd rejected it. By leaving it out there to fend on its own, of course they had to euthanize it based on this quote from their website:
In addition, interference by people can cause mothers to reject their offspring. In this case, park rangers tried repeatedly to reunite the newborn bison calf with the herd. These efforts failed. The bison calf was later euthanized because it was abandoned and causing a dangerous situation by continually approaching people and cars along the roadway.
I understand the circle of life and all that, but IMO if human intervention caused the problem, human intervention should FIX the problem.