The line between "user" and "bot" would get very murky very quickly. If you write a tweet and then use a bot to post it at a certain time of day, is your speech still protected, even if it was posted by a bot? What if you have the bot post it at the same time every day? What if you have the bot post it in reply to all tweets with a certain hashtag? Or just all tweets?
In each of those cases, the original speech is a user's, and the bot is merely following the human user's publication instructions.
In each of those cases, the original speech is a user's, and the bot is merely following the human user's publication instructions.