Great question!
You need to distinguish Steemit (a company) from Steem (a blockchain). The latter is a decentralized network that stores your content on a blockchain, while the former is a company that uses the data stored on that blockchain to profit from it (somehow). Neither of both own your content. Steem merely stores the content and Steemit merely shows your content and offers you a way to post more content.
In future we will probably see many more platforms to use Steem as their database backend and none o them will own your content.
For me, it is unclear what owning content actually means. You own the keys that can edit a post. You also own the account name that has posted the content. It should be clear that the post is available publicly and anybody can access/read it for free (on a global scale).
That said, you could attach a LICENSE to your own posts and to make sure you own your posts (what ever that means) but make sure that you don't ask for things that no-one can ensure, e.g. you can only read this post if you paid $1 to this account (that's simple not possible, technically).
I think the question is more do you retain control over the content. Since it is written to a blockchain, I would say it depends on how the blockchain functions. If you can delete your content later, permanently, from the chain, then you do still own it. But if you can't, then I can't see how ownership means that much. If the blockchain itself is considered a common asset of humanity, your content is effectively public domain.
With bitcoin, ownership is being able to move bitcoin to a new address. However, that doesn't delete the record of where that bitcoin has been--that can never be deleted. So while you retain ownership, or at least control, of your bitcoin, you do not own the ledger itself. Is Steemit content part of the ledger or is it an asset you can move around? Although to be fair even moving bitcoin doesn't destroy it. It can never be destroyed, just made inaccessible. So perhaps nobody truly owns bitcoin, they just control it for awhile. These are interesting questions.