It used to be a very large problem for the 2007scape streamers, I imagine it was solved with a mix of legally prosecuting serious offenders via de-anonymizing the bot networks, as well as a tighter feedback loop of reporting and banning them - effectively making it more of a hassle to setup than its worth, which is saying a lot because the types of people doing those activities have A LOT of free time.
IANAL, but at least on the /2007scape streams the viewbots were used to pump up numbers on fake streams linking to phishing sites (which presumably is an illegal activity in most countries).
For other content, it can be seen as a loss of income for the streamer if they are demonetized due to viewbots, and for those where it isn't - it is effectively ad fraud since Twitch revenue is based on those engagement numbers
