This is good in my opinion. The more the rejection, the more the developers will tend to flee away from arrogant Marketplaces like that of Apple's. It will also, in a way, probably force developers to try out other marketplaces like the Windows Phone Marketplace and the Google Play store. Now, I do understand 500px is trying to create a uni-platform experience for its users, but now that 500px isn't allowed to publis…
I think the 500px app shouldn't be too complicated running as a web app. As far as I can see, there's no big usage of iPhone hardware sensors which couldn't be easily implemented in a web app.
So maybe this is a way to go.