Linking youtube content is fair use and granted by the youtube license. Anything that is granted a license to use in any manner you see fit is ok to post on Steem. Steem isn't only about about creating original content, it's about creating work for the blockchain. The whitepaper, which few will read, goes into great detail about this.

Posting any content, of which I have a license, to the blockchain provides value to my followers is in and of itself "original" and provides "work". As long as you are not using content you don't have rights for all legitimate posts should be allowed and ignored by the cleaners.

If I take pictures from NASA of the moon landing and I spend the time to publish them on the Steem blockchain and my followers enjoy the format in which I have displayed it for those users I am doing work and the people who upvote are curating that work for the network.

Did I go to the moon? No. Did I take the pictures myself? No. Did I present them in a way that others can easily see and appreciate them? Yes. Is it OK for me to do that since it is public domain? Yes.