To everyone wondering how it's an antitrust issue, I think this quote right here hit the nail on the head.
Edit: On the other hand, I just looked up Roku and it's all Roku devices, I guess as long as I don't see an Amazon product on that first page that's fine. It's still weird you can't buy ads for your own product's very name though.
I can't read the full article due to paywall unfortunately, but if they didn't do a more thorough investigation I think someone might just have to, to make sure Amazon's not putting their own ads where they don't allow competitors to buy ads for products they don't actually own themselves like Roku, if they aren't then maybe they're just trying to protect the results given back.