This is a bad faith argument. There are documented cases of LLMs being able to reproduce, verbatim or nearly verbatim, source content; there have been lawsuits over this.
Any given output _might_ be slop and it _might_ be plagiarism and it _might_ be neither. It's stochastic!
It's ludicrous to ask everyone who has ever created content ever to continuously monitor other people's LLM output for plagiarism of their content. We know it happens. It's up to the LLM users to police it.