> advertising agencies are the best, most benevolent arbiters of speech.

Needs an /s

You get what you pay for. You're free to start a subscription twitter that hosts all the hate content you want. Platforms that sustain themselves on advertising are going to do what is popular with the public. Turns out most people, despite what some may think, are not hateful violent lunatics. There are plenty of services that host that content if you want it. Google is at your fingertips. Just don't expect people to like you or demand it be mainstream.