I'm not even sure what manipulation means here. What rule is being violated by writing a post which promises its payouts will be used to buy HIVE?

Even if it doesn't affect the price of HBD much (and I agree that adding a few hundred to 7200 daily isn't likely to change much), it does mean making a profit for Hive stakeholders. I'd be hard pressed to come up with anything wrong with that, other than possibly that the reward funds could be used for something better.