As to why the behaviour needs legislating: because the British social contract required it, because in British society they're willing to trade the right to say grossly offensive things to people, for protection against hearing grossly offensive things. Lots of reasons, it's fine for them, you don't often hear clamour to repeal the law, they don't have the same issue with it that you do. So who are you to tell Brits that they shouldn't have a law?