Apple seemingly has no way to determine if the phone has been damaged due to a hack. The fact is you CAN toast hardware by doing a firmware hack. Why would Apple offer support for that iPhone (assuming it had been fully bricked)??? If you do a full restore first, Apple seems to have no way of knowing if there was any damage done due to the hack, and therefore shouldn’t be obligated to except the phone knowing this is a possibility. The 10% restocking fee is normal within 14 days on an opened iPhone.”]
Source: https://gizmodo.com/apple-stores-voiding-warranty-on-hacked-iphones-302464