Back in the day, if you ran an IRC server, you generally ran the service laissez faire. Your chief concern was providing the service. If someone was sharing illegal content, that wasn't your responsibility any more than state governments sharing culpability for roads that facilitate robberies. And behavior? Who cares what you do, unless you're attacking the service?
Channels themselves could come up with their own set of rules, if they wanted. You want to restrict swearing? No problem. Only registered accounts? Fine. No discussion of politics? Who cares? If you came crying to the server administrators, because you were unfairly banned from #atheism/religion/linux for walking in and trying to have an honest debate, they'd shrug their shoulders. Not their problem!
What a ridiculous state of affairs to have the service administrators make a network wide rule: "No saying untrue things", or "Don't be mean, but only to certain groups of people". It isn't their job to decide what is true, or what is hurtful.
"Oooh, but bjt2n3904, the First Amendment doesn't apply to Facebook! There's this great XKCD comic where he shows mean people the door, and..." -- Yes. I get it. But the fact remains, the 1A is a good principle to adhere to, even if you aren't the government. Policing your own little channel of the internet is feasible, if you so choose. Policing the entire internet is madness -- especially when it comes to subjective things like "hate".
Everyone has a different level of tolerance for putting up with jerks on the internet. One person might feel they're being targeted with harassment, another might just call that "Monday". The solution isn't to have Twitter declare how everyone should behave, the solution is for Twitter to push that decision to the user, and provide them with effective tools for filtering and blocking people.