Because they are mismanaged, bison carry and spread brucellosis, a highly contagious, bacterially-caused disease that causes spontaneous abortions in cattle and other animals, and also poses a serious threat to human health. The goal of free ranging bison is a good one. It will remain a pipe dream until and unless the brucellosis issue is successfully resolved.
Quoting the authors above, “Wild horse advocates note that, while horses died out in North America more than 10,000 years ago, they evolved here. Horses aren’t an invasive species, they say, just a re-introduced one.”