When you upvote any content on Hive you get back 50% curation rewards. Hive SBI works exactly the same way. You commit stake for permanent upvotes on the account, and then you get back 50% as upvotes on your own account.

Putting it through the Hive SBI mechanism has some advantages over managing your auto-votes through an auto-voter, and some disadvantages.
At the end of the day, curation is curation. Just because you don't understand the mechanism doesn't make it vote-selling.