Even if it is on the blockchain, they reserve the right to not display the content on their centralized service/website. That's why we need a Memo application.
Ideally, we would need a program that can connect to existing nodes like Electron Cash does, and then index that data locally. This way the entire blockchain doesn't need to be stored.
Realistically, we will likely want people to host their own Memo nodes, thus allowing users to choose which nodes they trust and this way no data is stored on the user end.