Twitter has shown that they can and do exert control over what's posted on their platform. No service provider immunity for them, they're aiding and abetting this. They need to get sued, too.
The law states they are not liable as long as they do not demonstrate effective control on what happens with their service. They have demonstrated that they do indeed control what happens on their service, and what gets posted there.
However if you modify the retweet at all then you too can be held liable.