Well, the problem is that using Artificial intelligence to create post is that it widely differs from the classic definition of plagiarism and the way it has been created for most of the history of humanity. You know, until not long ago it implies the copy/pasting and small modification of information which did not belong to us but try to fool others to believe it does.
If you post AI-generated text without crediting the AI, how is that any different from plagiarism? You are attempting to take credit for something that you didn't write.
The main problem is the origin of the information, if the AI was legally recognized as an author, then it would easier to accuse someone using it to generate content for their own purposes to be a plagiarist, on the other hand, it would also imply the AI itself is committing massive plagiarism.
For the AI to be committing plagiarism, it would have to be demonstrated that the AI is directly copying words from another source. It does occasionally do this, but not always.