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Sure, any front-end can determine what users do or do not see. But in the end, as the open-source code that underlies front-end development proliferates, users will be able to gravitate towards services which align with their needs and values. In the future we should see competition among applications for users that wish to access the Steem blockchain, much like individual websites compete for our use and attention on the internet today.

They could, but I don't think they are currently.