Here in Montana most people have bear stories to tell, i.e., those who spend an appreciable time in the outdoors year in and year out. I have had my own encounters with both black bears and grizzlies, but I won't bore the readers with the details, other than to say some were very close calls, and I was very lucky at times.
Back in the 1960s, the noted Yellowstone bear researchers, the Craigheads, examined one big boar in the fall which weighed 1,140 lbs - big enough by any bear standard. The grizzlies are no longer endangered but expanding into historical regions out of the mountains, where they eventually get into trouble and have to be relocated or worse.
For reasons not fully understood, Glacier Park grizzlies for a while seemed to be more bellicose around humans than those in and around Yellowstone. This last fatality is number 3 for Yellowstone in the past two years.
Bear news travels at warp speed in Montana, and it appears that the couple near Canyon weren't really up on bear protocol. First, they did not have bear spray with them. Second, when they first saw the bears, they retreated for a while and then returned moving on the trail. When they saw the sow coming after them, they ran away. Perhaps Usain Bolt could pull that off, but not folks in their fifties.
My experience with grizzlies is that often their charge is a bluff. Standing calmly and facing them will put your heart in your throat. But a mama grizzly with cubs is one of the world's most unpredictable and dangerous animals. Jim B mentions numbers of hikers adding a measure of safety, and I believe he is correct. The park service will probably require a group of four for back country hiking and camping one of these days.
Solo hiking is a recipe for trouble. Add the sub-adult male into the picture with mama, and anyone who hikes alone or in twos will eventually wish they hadn't.
Yellowstone has had a very late spring, and the bears are out foraging everywhere. On June 22 Edna and I took a drive from the marina to Sedge Bay where reports of a big boar hanging around took us there. And sure enough the lone grizzly, a big male, was grazing like a cow. We took pictures from the road (inside our car) at an estimated distance of 40 feet. He paid no attention to us at all.
What was most interesting was after a while he ambled across the road to the west side and worked his way past a small group of four mature buffalo bulls, and neither species seemed to be the least bit interested in each other. But there are recorded cases where grizzlies attacked big bull buffalo and were bested in a life and death struggle.
What this illustrates to us is that the inexperienced person witnessing this scene will reach faulty conclusions about these animals. Buffalo have killed more Yellowstone tourists than grizzly bears.
This is a tragedy for both the hikers and the bear. But Yellowstone isn't a zoo. IMO the assumption of risk must be borne by humans who visit this wild country. John