Do you mean why wasn't twitter kicked off AWS for users making calls for violence? If so: because Twitter has a moderation strategy in place that clearly isn't perfect but has proven to AWS that they're taking it seriously.
If you actually meant why wasn't @blcklivesmatter kicked off twitter I guess I'm not sure what relevance that has to Parler being kicked off AWS. But my first guess would be: because they deleted the offending tweets and stopped making repeated posts calling for violence?
I don't use twitter, do you have some examples of recent @blcklivesmatter tweets that are actively calling for violence against elected officials that haven't been moderated or removed?