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Not sure why people are getting worked up.
Unless bison are very different to cattle the cow would have been close by and got itself involved. No mad bison cow means calf was abandoned. Abandoned calf means dead calf. Along comes tourist and fucks with calf. Calf ends up dead. In the end the tourist left no trace.
The only thing making the tourists getting involved not a good idea are the quarantine rules preventing the calf being removed from the park. If the calf could have been removed then picking it up was the right thing to do. As it is picking the calf up was inconsequential.