Your public domain assumption doesn’t have to apply to others, legally or ideologically.
Data ownership does exist in the Fediverse, in fact it is one of its selling points that you can set up your server and own the data instead of using a surveillance capitalist SaaS that stores, manipulates and imposes legal rights over your data. Applications like Mastodon do send a federation request to other instances to delete data if submitters want to. Additionally, some users put licenses on their profile that might have restrictions (i.e: CC non-commerical, etc.) on what you are legally allowed to do with the data.
So no, accessing the data is not the same as using or processing it for many people, legally too in several parts of the world. Also, “innocuous curiosities” label is entirely subjective.