If you had an actual contract, it would be a different story. Your product would still be not more than some UI to some walled garden, and nothing particularly valuable, but at least you would be officially permitted to do so.

So you basically sold something to Apple users that you don't actually own, and that is completely out of your control, and that was always in danger to just stop working suddenly.

And, on the other hand, I'm sure you've implemented a (for a hobby) decent app, and maybe you actually brought a handful of new visitors to them (which wouldn't have just used their website if your app wouldn't exist). But do you really think that at Yelp they are sitting there and just think how they can return the favor?

They basically think "well, that funny dude has now played enough". You never were a partner for them. Just what people call " useful idi*t" (sorry, but that's just the term ^^) at best. They haven't talked about you at their christmas party!