Thanks for your comment. I see a couple important differences between this and promoting a post:

When you pay to promote a post, that doesn't in and of itself take anything out of the reward pool. Users have to click on the promoted tab, see your post, then manually choose to upvote it.

Also, when you pay to be in the promoted tab, that SBD is burned. In other words, it's taken out of the total supply, and it makes everyone else's STEEM more valuable.