Yes its a danger to go out into areas with large predators. Bears, Cougars, alligators, sharks and wolves to an extent all can see us as a threat ot food. it doesn't stop people from moving onto the water in alligator country it doesn't stop swimmers and surfers from going in the ocean.
Your right Yellowstone is a dangerous place. It was set up to preserve both the natural wonders and wildlife. as a way of showing people what the west looked like before we tamed it. Its also an active geothermal area and more people are killed or injured by the well marked hot spots then by the wildlife.
As far as the wolves killing elk look at the herds in yellowstone they are healther and more robust now then in prewolf days. Wisconsin has wolves now too. Ours wandered in from canada through minnesota we now have to live with them. Deer hunter blame the wolves for lack of success but won't blame the harsh winters or year of over grazing by out of control deer populations. We also have a large population of black bears while not as dangerous as grizzlies they are still unnerving when they hang around backcountry campgrounds looking for scraps, they are alot quieter and sneakier then you would think.
The bears and wolves spreading faster then first believed yep they do that. they are looking for food, dens, and mates. Its the nature of the animal. Its the same thing humans do.
Your two encounters with Grizzlies where warning charges. Its good they "lost Interst" otherwise we wouldn't be having this discussion. You entered an area that the bear was using, it was protecting a food source or its young. it would of used other body lanuage to communicate its discomfort with you before mock charging. such as turning sidways to show how big it is, yawning to show its teeth, swaying back and forth. If your worried about being attacked maybe it would be smart to learn the classic warming signs animals use to show discomfort with a situation they always warn before they attack. Unless they are seeing dinner then its part of the dangers of going into the back counrty.
Idaho has more than likely had pockets of Grizzlies and Wolves surviving before the attention to reintroduce them to wilderness areas brought greater field studies. it was belived they were moving back in from canada and montana or that they had survived in isolated areas. I would personally be more worried about Moose during the fall more people are killed and injuried by agressive moose then bears. Ask people who spend time out in the bush in Canada and alaska what animals they are most dangerous and moose will be on the top of the list.