Capturing and killing Buffalo prevented their extinction.
If I would have told the makers of this movie that back in 1971, they would have thought I was crazy. However, the truth is that did prevent them from going extinct. Having them rome wild acrossed the country would've ensured that they died off. Ironically, the kids in this movie were doing more harm to the Buffalo species than the grown ups who were shooting them in captivity.
Since there was a demand for buffalo meat, it gave the people who owned these animals incentive to have them reproduce and increase their supply in order to fullfill that demand.