Holding SP does mean you're a shareholder in the value of the Steem network though. While this doesn't mean you have any share of Steemit, Inc it does mean you own influence to the actual Steem blockchain network and can influence it's decentralized nature by voting for witnesses that are aligned with your thought process. While this is certainly not ownership in the company Steemit it is a share of the network's operations and influence towards it.