I don't see anyway how can this be prevented. People who would love to see Steem being close would just have to post copyrighted material and then create a law suit.
I don't see it as making Steem liable, maybe dtube could be seen as liable but I don't think so either.
Most of the time torrent websites get closed because they don't comply to taking down some material but dtube can't do this. Dtube could be seen as facilitating piracy but their technology is neutral.
Then again dtube will have to comply to take down notice just like Steemit Inc is meaning they will have to remove certain content from showing on their website if the authorities ask them.
I dont think the authorities will care about the fat it is not possible to remove content from the blockchain to be honest. As you say, maybe they are ok for the blockchain/services to prevent showing copyright material to the user, maybe this shall be build into the blockchain API as well, so that no frontend is able to get to content anymore that is required to be blacklisted/taken-offline by the authorities.