Even if it is linked externally, Steemit serving the links could be liable for using copyrighted content in their business. Although it is on the Steem blockchain, Steemit doesn't have to serve the content. They should implement a copyright compliance team. Steemit is a centralized entity and thus is liable.
That is unless the user is downloading a client and the user is deciding which content from the blockchain the user wishes to request and view.
So that's why this guy made $40,000 on this one single post.
https://steemit.com/piston/@xeroc/piston-web-first-open-source-steem-gui---searching-for-alpha-testers
Can the blockchain handle every user downloading the content in real-time from the witness nodes
That would explode the bandwidth requirements. Also probably open the witness nodes to DDoS attacks. You must have some replication nodes then are users going to pay them with per access fees?