500px's business model isn't based around an iOS app. It's supplemental to their service. View your comment in the context of the following headline:

  Apple Pulls iOS Flickr App from AppStore

Building an App for iOS isn't the same as basing your business model around playing in someone else's sandbox.

Because unfortunately, the shifting sand is made of gold :(

Now consider, how many apps have been accepted vs. apps rejected. Google does not even offer an option to sell apps where I am.

Google's store is not the only way to get code onto Android phones (which is why I bought one, after rejecting iPhones).

Or, really, people build on this pile of sand because it's more valuable than not doing so. There is enough money to be had in the iOS ecosystem that even when you take into account the risk of being dinged for something stupid like this, it's still an expected net gain for many people to release an iOS app.

You missed the obvious reason:

Money.

Lots and lots of money.