Essentially it works like it should anywhere else: If you don't publish source code and someone complains, you're out. In an ideal world Google Play would hold itself to the same standards.
However, the App Store does impart restrictions which are incompatible with GPL, so it's not so much that Apple doesn't allow it as it is that GPL doesn't allow it.
Apple has no official statement on this of course, but we know how they feel about GPL code given that they've removed every bit of it from their OS over the last few years.
They haven't been removing GPL code actively, they decided not to take the risk of using GPL version 3 software, so they've let the respective versions of bash, rsync etc. linger at their last released GPL version 2 releases.
E.g. Git is on GPL version 2 still (and probably forever), and Apple continues to update that in a relatively timely fashion.
You attributed the incompatibility to the GPL. Apple created the incompatibility. Apple could easily fix it. The FSF can't change existing versions of the GPL.
https://forums.developer.apple.com/thread/18922
Essentially it works like it should anywhere else: If you don't publish source code and someone complains, you're out. In an ideal world Google Play would hold itself to the same standards.
However, the App Store does impart restrictions which are incompatible with GPL, so it's not so much that Apple doesn't allow it as it is that GPL doesn't allow it.
Apple has no official statement on this of course, but we know how they feel about GPL code given that they've removed every bit of it from their OS over the last few years.