The answer is not brucellosis. Since 1917, when the disease was discovered in livestock, there has never been a documented case of bison transmitting brucellosis to cattle in a wild setting.
The brucella organism lives in the reproductive system. The bison clean up their afterbirth and scavengers quickly eat the rest. It only takes four hours of sunlight to kill brucellosis.

In the Horse butte area there is a thirty-day buffer time before cattle are brought in. Why pick on bison when other wildlife in Yellowstone carry the disease in much higher numbers?

The Gallatin Land Agreement of 1926 set aside public lands surrounding Yellowstone so that wildlife could have a winter foraging ground. When the bison cross into Montana their natural right to survival is taken away.
The DOL run them back into Yellowstone or into a capture facility and then to slaughter. The whole process is life threatening to other wildlife as well.

On February 14th, a DOL helicopter, running a siren and firing explosive rounds, hazed fourteen bulls (who can't transmit the disease) along the Madison River at least four miles into Yellowstone.
Elk, moose, eagles, trumpeter swans, and other wildlife fled in every direction. In the Duck Creek area, the helicopter hazed 1 bull bison into the park.
Six bulls that were in the park and have been in the same area for at least two months were hazed miles further into Yellowstone. Cattle are never present in the areas where the bison were.
Who gets to foot the bill for this madness?

You and I.

"The greatness of a nation and its moral progress,
can be judged by the way its animals are treated."
Ghandi