Why? European Bison still exist without any need to cross them with American stock. There is a herd in Poland and Belarus.
The gene pool is small. All the ones alive are descendents of the ones that were alive in zoos when the wild ones went extinct. They need to diversify their genetics, so breeding in some American bison genes, here and there will help to protect the remaining ones, in case of some disease that could wipe them all out.............