[gpm]

> why wouldn't they just keep doing it server-side

Because they want to scan files that will be encrypted on your device and not transferred to the server as plaintext. No doubt they'll continue to scan unencrypted files server side.

> no one asked Apple to do this.

If someone asked them to it would turn it into a warrantless search with Apple acting as the state agents to search your possessions. It would almost certainly result in the evidence being tossed by any US court. This kind of warrantless search only works if Apple does it of their own accord, instead of on the governments behalf.

Note that this doesn't apply to searches done on Apples servers to data you uploaded, there it's their possessions that they're searching, not yours.