The key word that I have a problem with here is not "mass", but "surveillance". "Surveillance" implies a human is directly observing or remotely monitoring an individual to "catch" them doing something wrong. That's not at all what's going on here. No one at Apple knows anything about what's on your phone unless you choose to upload illegal content away from your phone and onto their servers, as has been explained ten thousands times.
Apple's own account shows where the software is, and on what platform the checks take place? Its on YOUR hardware, not in iCloud. The software is not on iCloud, it will be on your device using your hardware that you own,, that you pay the electricity bill for, that you pay your internet supplier for etc. etc.
A few years back courts took a view that emails on a server were not necessarily private, whereas emails on a device at home were, and on occasions it meant no court order was required to get details of information stored on a server, hence many people immediately delete email from servers.
Technically and legally it would have been much easier if Apple had suggested that checks on child photos were taking place in iCloud as its their servers (or Googles) and left the operating systems alone, as there is literally no need for that unless it is FOR SURVEILLANCE. So why didn't they just have iCloud check rather than via our hardware, using our processing power, our electricity and equipment we have paid for outright?
On their own servers they could legally do that as they own them and their terms and conditions would apply and then it would be a choice for users as to whether to use iCloud or not, but I don't see this latest situation helping them sell iCloud+ and absolutely the wrong move to make it part of operating system on devices they have sold to consumers, especially on their privacy platform.
However they have chosen to enact SURVEILLANCE via software embedded in our hardware. Hardware we have paid for, we pay for electricity for, and on the basis of a performance not curtailed by surveillance, surveillance that will not help the fight against child abuse anyway, as they've telegraphed how to avoid being caught, making the whole system a complete farce.