I really dislike this argument. A lot of democratic countries have defined hate speech. In the US, individual companies define it and moderate it as they see fit. In other countries, their legislators and courts define it. The US has defined lots of difficult terms to define already.
>And why ban speech that is actually legal (even if highly offensive) anyway?
I mean if we're talking about why companies should ban it, I'm sure they have a variety of reasons that range from bad PR and loss of revenue, to the founders/owners/employees don't want to build a product that's used for violence.
If we're talking about at the society level, because it threatens democracy, peace, individuals safety
>And if it's worthy of banning on a major platform, why not just make it explicitly illegal?
A lot of democratic countries already have. UK, Canada, France, Germany, New Zealand, Sweden, Switzerland (and yes, they managed to define it somehow): https://en.wikipedia.org/wiki/Hate_speech_laws_by_country