For Meta to analyze your data they need to either scrape it (legally questionable and scraper bots are commonly blocked on server level) or work with a local copy. By federating with them you are allowing them to legally make a local copy of all the posts of the instance.
Newspaper articled are often also public, yet google got sued (and lost) because they were scraping and analyzing them to put previews in their search results.
Just because something is public doesn’t mean you can just take it. Copyright still aplies.
Defederation does stop legal use, and Meta is already in enough legal trouble, especially in the EU, that they are unlikely to blatantly pirate user contributed content from sites that defederated from them.