Physically unlocking the phone breaks the rules regarding forensic data retrieval as well, since the state of the phone has now been altered, how is the court to know that the FBI didn't plant the details.
This is why backup images are made first, which is not possible with phones when they are locked... the backup images are operated on when doing digital forensics, so that the result can be reproducible by a third party.
How does this violate the rules? He's already signed a waiver to his 4th amendment rights, so no court order is needed - he's essentially given them full control over his own property, including his phone (which I presume would include accessing the data via adb instead of the phone screen).
Because then the phone is no longer in the original state as when the defendant turned it over, and the FBI cannot prove it did not alter the contents of the phone (as images cannot be made while the phone is locked).
This is why backup images are made first, which is not possible with phones when they are locked... the backup images are operated on when doing digital forensics, so that the result can be reproducible by a third party.