But if they're looking for incriminating evidence on your private property (i.e. on-device scanning) then they do need a warrant. It doesn't matter if a copy of it was also given to a third party (i.e. uploaded to iCloud) what matters is where the actual search takes place.
The authorities aren't doing the scanning. You will be made to agree in the fine print to let Apple do it when iCloud sync is enabled. If they run across evidence of a crime then c'est la vie.
