"Intellectual property" is mostly a capitalist idea. To give a direct example of the harm caused by LLMs using others' work without attribution or credit, there is the recent case in which one of the models was being portrayed as finding a mathematical proof which was actually in it's training data. The original authors will not get the credit, academic support, exposure and funding they would have gotten otherwise.
Also, even if information is not "owned", it's epistemology and source are an important part of it. Consumption by LLMs degrades the information by removing such metadata, and even worse hallucinating sources.