Can the border guard go to your house and retrieve something, bring it to the checkpoint, and ask you to do something with it before entry? No. This is out of their legal authority.

Also, you could set up a system where the phone cannot restore the backup on reentry. Perhaps a single use restore key that you use at your original destination, so the restore cannot be performed again until you return home and generate a new code. This evades the (flimsy) charges that were applied in this case.

The best option, however, is to bring a blank disposable device, restore from backup at your destination, then discard the device before you cross the border again.