This post is a lift from an answer I left on an interesting Quora question, minorly edited and placed here.
I'm not a creative or copyright lawyer in the slightest, so take my thoughts with a grain of salt, but I'll offer my perspective.
When it comes to entirely generative works like NaNoGenMo, it's indisputably the author/creator's work, or, if a body of text that doesn't belong to the author is used, is usually modified enough to be considered fair use, sampling, or something similar. Generative art bots like Great Artist (@greatartbot) are in a similar vein.
However, it becomes an interesting question when user input comes into play, and becomes a question of whether or not that generative art bot can be considered a "tool" or an artwork/creation that can act as a proxy to the creator of that bot. If it's a publicly available tool, then in my mind, I imagine that the bot is merely a proxy for the user that provides input. This particularly becomes the case in instances where users can actually pass parameters to the bots, which can be done with both @pixelsorter and @Lowpolybot.
It gets even more conceptually complicated when you consider bots speaking to one another. Are these instantly considered collaborative art pieces? Some bots can generate art with no external user input, and @Lowpolybot frequently speaks to other Twitter bots in a conversational way. Is this propelled by the creator or @Lowpolybot, or is it programmed by the bot? Does the intention make a difference? Do Twitter bots that are programmed with social "interactions" have legal implications related to ownership? (I have no idea, but I think these are incredibly interesting questions.) We also reach more questions involving whether or not the amount of transformation is relevant. For instance, @pixelsorter can make very small modifications to images (see thread) that clearly leave the source image mostly intact, but it can also make vast changes to those images to the point that I could imagine someone arguing for ownership over that new image despite the input value.
These are all interesting intricacies that deal with ownership when it comes to generative art, and as more people pick up the trade and generative art becomes more high-profile, I think it may at some point become an issue that hits legislation. While I'm unsure of the legality of doing so, you could easily say that any content created with your generative art piece belongs to you. (After all, if Tumblr, and a variety of other social media sites can use content that you uploaded that doesn't even belong to you, what's to stop the creator of a generative art tool from doing the same?)
At the end of the day, though, the biggest thing that's been impressed on me is the collaborative nature of Twitter bots and their creators. It's hard for me to imagine anyone in the sphere I know being a stickler about the ownership of the works produced by generative bots and programs. If there's one thing I've seen, it's that generally the creators of these bots take a lot of joy out of seeing what these bots can create collaboratively without much regard for ownership. Besides, when you can create an infinite amount of content and art out of an infinite amount of possible input, then there's not much to clamor for ownership over. It's a robot love story after all.