The problem is that, as it is now, post promoting could be used to lower someone's reputation drastically, probably to the point of having them completely censored after a few attacks.
Imagine a situation in which a relatively low quality post gets promoted for $5k, making it top promoted. It wouldn't be too long that large SP holders with high reputation to downvote it excessively to make it disappear from its position at the top of the "promoted" page (as recommended here).
Since Flagging and downvoting are currently the same, the promoted user's reputation would take a significant hit, that being completely out of their control. In the short term, those types of attacks are expensive and quite unlikely. Still, if a whale conflict emerged, it could be done anonymously with no repercussion for the attacker (aside from some thousand SBD loss - a small amount for some).
This is already something that we know to be possible (have you ever witnessed the full power of @dan's downvote?), but low quality posts wouldn't generally make it to the trending page in the first place. If it did, the attacker would be risking paying the blogger directly and exposing themselves through upvoting, making themselves easily identifiable as a bad actor. With post promotion, any post can now reach an undeserved ranking and visibility in the "promoted" tab instantaneously and anonymously.
You may think the incentive is low, but in the long term, large advertisement companies could join the site. Imagine a Coke vs Pepsi scenario, in which Coke censored Pepsi completely with repeated attacks at a fairly low cost.
Although implementing it might require a lot of work on the dev side, I think this is another point that supports the separation of downvotes (to lower payouts) and flags (to lower reputation/hide dishonest content).
Any other suggestions?