The argument is a bit pot/kettle TBH.
Everything you post on an internet forum (so MN here) is owned by the website. This post will have the copyright owned by MN, not by me the original author. I have no say in how this post is used in the future.
They are complaining that it is unfair that AI companies are profiteering off of their content, when in fact the content is user-generated, but MN own the copyright.
The business model of MN (as well as the advertising) is on the ownership of the copyright of the user-generated content and AI offers them an opportunity to sell that content.
I have no problem if AI want to use this post, but as soon as I click Post, I no longer own the copyright, MN does. I have no control over the use of this data, and now MN are claiming that they have no control over it.