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When someone makes use of a service and doesn’t pay afterwards that is considered to be theft even if the provider hasn’t been deprived on anything. For example, if I snuck into an art gallery without paying I won’t remove anything tangible since the gallery’s overheads and running costs were fixed long before I arrived.
A better word would be copyright infringement if the AI is making use of other works without a license or other permission. Based on my reading of the article it appears those involved only fed the AI works in the public domain or works that they had created themselves. The letter of complaint appears to be signed by artists who are unaware of these circumstances.
There are cases progressing through the courts. If the courts rule that copyright has been violated by the AIs under current laws then we won’t need to create a new offense or expand IP laws currently on the books.
A unique work of art I guess since it’s unlikely anyone would be able to replicate the prompt in order to get the same results.