That is just not true. Broader definitions are actually very useful when you're trying to describe general principles (like in law) — using a word that's too narrowly defined can limit the scope of what you're saying.

That's the difference between a principle and a definition, law has both - criminal offenses for example are more defined than the principle of tort, the more you want something to be useful as an accusation the more narrowly defined it needs to be or you just end up with a moral panic.