"Unauthorized practice of law" only applies to people, not tools. AI is a tool. DoNotPay was not selling legal advice, only a tool to understand law. It is no different if they were selling a code book, or other text that the defendant uses himself. I think the real fear is that AI will supplant the entire legal profession.
The legal profession went through a similar struggle when Nolo published software that could draft basic legal documents by filling in the blanks. Nolo won.
You are making the assumption that a company advertising a robo lawyer isn't engaged in unauthorized practice of law, which is rather odd since I bet the Nolo books don't hold themselves out as a lawyer, robo or otherwise.
You are also making the assumption any tool is allowed in a courtroom, which is obviously not correct. You wouldn't be able to use a nolo book while testifying what you witnessed at a crime scene, either.
The legal profession went through a similar struggle when Nolo published software that could draft basic legal documents by filling in the blanks. Nolo won.