Again, you're right that their "brucellosis-free" status is very important for them to keep, I don't disagree there. And I understand the mechanism of transmission - it can transmit between them. But there is literally no reason why they should ever be sharing the same grazing ground to begin with. These aren't "seemingly" drastic measures they're taking - they are 100% drastic and unwarranted. There has never been a case of transmission between the two species naturally because it would never happen unless someone was allowing their cattle to graze on protected land.

That's where the problem lies though - they can't graze on the same land because of the threat of brucellosis, but ranchers want the land the bison are currently on. They want to cull the bison because less bison means less grazing land for them and more for the farmers to then use without the threat of their cattle contracting disease. And less bison also means less likelihood that the state will protect more land for them to graze on, which is once again great for cattle ranchers. They're using fear-mongering tactics to advocate for killing bison and getting more land, not fighting the good fight to protect their way of life and the public from disease.