> If that profit comes from advertising, and critically, if users don't expect advertising in their free LLMs, because they didn't see ads in generated output in the past, that will be very insidious.

Are the free LLM providers offering their service with a contractual obligation to the users that they will not add advertising to the outputs? If not, how is it insidious?

What definition of insidious are you using per https://www.merriam-webster.com/dictionary/insidious?