There are many things that go wrong with the App Store, but you have to realise this is a ludicrous assertion. Pasting from clipboard is an entirely different class than rootkits, backdoors, trojans, and other malware that have a vanishingly small chance to go through the review process itself.
This is defence in depth. The sandbox helps, but it’s not magic. Without review, publishing an app with a sandbox bypass, or simply abusing entitlements, becomes trivial.