I haven't seen the contract between Apple and (say) HBO, obviously. But it would seem Apple would be well within their rights to advertise whatever apps they choose (likely their most popular), on whatever ad platforms they choose (Google being one of, if not the largest) and direct users to the App Store to download.
I'm pretty sure that's how every retailer that carries other brands (i.e. Target) does it. They have a hot item (say an Apple Watch), they advertise the crap out of it, getting paid for clicks, and then direct users to their store - be it brick and mortar or online - not Apple.com.
I'm not defending Apple here, at all, just not seeing the difference between what Apple is doing and what other retailers do. If I know anything about these forums, it's that someone here will enlighten me.