They're free to ban bots. That would clearly be a viewpoint-neutral restriction on content/behavior. "Using a bot" isn't a viewpoint, and banning bots is not a viewpoint-based restriction on user expression.

This is a very well-explored area of law, and nobody is confused about the definitions of the terms used here. Even the government has reasonable authority to restrict speech based on time, place, or manner (e.g. by disallowing amplified sound systems).