Yellowstone National Park sees 3.5 million visitors a year, most of whom come to see the habituated wildlife. The wildlife and the humans are used to getting close to each other, sometimes they get a little too close.
Before anyone gets critical of the people on the bridge I'd remind you that this is why people come to Yellowstone, and the Park Service certainly encourages people to come view wildlife.
In the wild, bears run from people. In areas of high concentrations of houses in the urban wildlands interface or around campgrounds and in National Parks, bears often become very comfortable around humans. People love large predators, black and grizzly bears, cougars, wolves.
Ordinarily wildlife takes off at first sight of humans. Habituated wildlife has become accustomed to being in close proximity to many humans and doesn't flee. This is great for photos, and tourism.
Unfortunately this skewed view of wildlife remains and is reinforced. Can you imagine this in the Zirkel Wilderness? Of course not, you'd be lucky to ever see the numerous black bears at all, they run from humans before you ever see them, just like they are in the, ahem, wild. The most popular youtube videos of wildlife are the equivalent of puppies and kittens, and the very most popular have wildlife acting with assumed human type motivations.
Humans are herd animals, and very domesticated, similar to cows or sheep. Wolf-watchers Lamar Valley Yellowstone.
For those who think there might be some sort of danger, biologists commonly tag, collar, and take specimens from black bear cubs while mom looks on. A griz would maybe disembowel a couple of photographers, black bears except old boars have a deserved reputation for timidity.
One of my favorite meme's of the clueless is how humans are evil and wildlife innocent. Black bear cubs typically experience high mortality, even with abundant food only half make it past the first year and the majority of deaths are caused by wandering males killing and eating the cubs to make the sows come into estrus again and to propagate their own genes. I certainly can't imagine half of human children that die being killed and eaten by males not their fathers.
Recently the infamous tiger temple in Thailand was shut down. They had many tigers and for a fee one could have one's photo taken while lounging next to or on an unchained tiger. It was very popular.