When we were planning to get a dog we read a few books & one had a chapter contributed by an American who said you should get their tails docked because otherwise they’ll injure themselves by hitting things when they wag them. It’s weird how these things become culturally normal. Over here tail docking is illegal except for certain working dogs, and even then, many vets are reluctant to do it. I don’t know of any dog who’s injured its tail by wagging it too hard. It’s certainly rare enough that chopping all their tails off seems an extreme way to deal with it.