Bears behave differently in places where they are used to a lot of human activity vs places where they are generally left alone. They learn and adapt. For example, in the Adirondacks in NY, bears have become adept at recognizing and grabbing human food from "bear bags" (food hung from a tree, theoretically out of reach on a limb that won't support the bear's weight) so that they have gradually had to adjust the rules for food storage to keep ahead of the bears, which can be pretty clever and tenacious. The bears there have learned to open some types of bear canisters, so a bear can that is permitted in some parks might not be acceptable in others.
In parks like Yosemite and Sequoia National Park in California, there is enough human activity that bears have learned how to get food from the sealed trunk of cars (through the window, through the back seat). They have a good sense of smell and they will be able to detect where the food is being stored. My guess is that if you have a small tube of toothpaste in the tent but a large amount of food in the bear can they would be more attracted to the stronger smell, but considering that you are supposed to keep the bear can away from your tent, the lighter smell might be enough to attract them.
It's better to be safe than sorry, not just for yourself but for the next person who comes to that area and also for the bear. If a bear becomes aggressive and attacks humans due to that human's carelessness, that bear is likely to start looking in more tents and packs for easy food, so in the US the wildlife services hunt down and destroy bears that attack people, even though it is almost always that person's fault and not the bear's. (And if you break food rules in a US national park and cause a problem with the wildlife, you will be fined heavily in addition to whatever property damage you have to deal with.)
It isn't always about fatalities that have occurred -- it is about preventing problems. In the US, many national parks (Yosemite, Yellowstone) used to have big garbage dumps that attracted the bears and became an attraction for people to come and watch the bears. This was bad for the bears and encouraged them to look for humans to provide their food source, which caused more aggression and other problems. In Yosemite, of course they want to avoid fatalities, but not just for people, also for the bears. (Here's a good discussion of this from Yellowstone.)
Regarding bear bells, it is true that most bears will not generally approach humans. The point of the bear bell is not to frighten the bear away, but alert it that something that isn't prey is coming down the trail. Making a lot of noise as you walk (talking, walking heavily, etc) will have essentially the same effect, so a bear bell probably isn't necessary unless you are hiking alone and/or otherwise being very quiet.