Depends on what they did. If Twitch lets, say, the Unibomber stream video, it looks bad to advertisers (Twitch's customers). They don't want to be associated with that.
It all makes sense when you look at it from a money angle rather than an ethical angle.
They wouldn't blame Twitch for what that person did, but they would blame Twitch for continuing to host them on their site.
For example: if Twitch hosts a well-known sexual abuser and groomer who's known for taking advantage of vulnorable fans, then people will be rightfully upset with Twitch for giving them an audience.
This is why Ryan Haywood got permanently banned from Twitch. Some of his victims and many of his former fans reached out to Twitch and said "hey, this guy is an abusive and manipulative piece of shit who will take advantage of vulnerable fans if he gets the chance. Please don't provide him with a megaphone and a potential audience."
People can, and do. At any given moment you can probably find at least one hashtag campaign excoriating Twitch for having not banned some streamer for something for other.
The context is that the last six months or so have been a shitstorm of prominent streamers caught in various forms of grooming, sexual assault, and so on. Much of this has happened off-Twitch, but it's certainly affected the reputation of Twitch to be associate with such.
It's no different to the owners of M:TG banning people for serious misconduct, for example.
It all makes sense when you look at it from a money angle rather than an ethical angle.