Only if you use iCloud photos, so you do have a choice.
In practice any major cloud provider is going to or is already doing this. We need a better regulatory approach, it isn’t practical to put the responsibility on providers.
"On device scanning" and "only if you use iCloud photos" doesn't make any sense does it? If it's really the case they're just preparing the ground for the next step which they hope won't get as much publicity...
Probably because they are also adding a feature to parental controls which when enabled checks images sent to the phones of your children 13 and under and if the image matches a known bad image they give the child the option to accept it or not, warning the child that if they elect to accept it their parents will be notified and given a copy of the image.
That has to be done on the phone because Messages is end-to-end encrypted. If they are going to have to have hash matching on the phone anyway for that, it makes sense to also use that for checking images that are to be sent to the cloud.
They've said the on-device scanning is done only when you are using iCloud Photos. So it's on-device scanning but only when you are deciding to share your photos with Apple... At least that's how it is for now.
"CSAM image scanning is not an optional feature and it happens automatically, but Apple has confirmed to MacRumors that it cannot detect known CSAM images if the iCloud Photos feature is turned off."
We won't get one though, because the "think of the children!!!" crap is extremely pervasive and anyone going against it will be smeared as a pedo defender.
It is unrealistic to expect corporations, which exist solely via regulations of various governments, to act as if they are immune from government regulatory control. In fact, a lot of negative press for FAANG (add extra letters as needed) specifically makes the point that they are acting as if they are bigger than any government.
In that mindset, the answer is absolutely to combat and to attempt to change broken laws first.
Also, it isn't just "think of the children". For instance, there have been some _terrible_ proposals under the banner of Right to Repair. People tend to not want to invest the time in understanding the ramifications of the actual proposals, and instead vote for or against the concept. One of the reasons ballot measures are both empowering and terrifying.
Good regulations take time and care - and generally, less is more.