It's not quite as slimy as, say, Samsung paying for fake grassroots badmouthing of competing products [1], but it's little different from fake user reviews on Amazon, unless the blogger discloses upfront, in a very visible way, that it's a paid-for post.

Nothing wrong with rewarding developers for porting app to your platform, there is nothing deceptive or otherwise unethical about it.

1: http://www.idownloadblog.com/2013/04/17/samsung-fake-web-rev...