Well, the blockchain is basically just the database, and Steemit is the application which fetch data from the database and display it in your web browser. Steemit have computer code that says something in the lines of "when I come across downvoted users or posts, hide that data in the web browser". But other people my chosse to build websites that fetch only those downvoted posts, only posts tagged as "science", or in the worst case, only posts containing childe abuse material.