It boils down to this: If you can prevent [some organisation] from potentially destroying civilisation, how much effort would be too much effort, and how much uncertainty is too much uncertainty?
For most, there's a trade off. If someone believes that the technology is sufficient for any country to implement a brutal civil rights destruction campaign, and that this is 50% likely, and all you need to do is upload some harmless images to your icloud to thwart it, why wouldn't you? For example, maybe a certain political party could regain power in a massive way and start doing away with gay rights, locking up anyone with pictures of men kissing each other on their phones. Of course, other countries have other types of extremists that look like they might take over government, or have already taken over governments. In those countries, tools like this are already in place, and this new one could be very powerful.
So if you could upload some innocuous images and save hundreds of lives in 20 or more countries in the world, even if you don't fear for your own country, would you?
Assuming you said yes, the question is, could everyone who agrees give Apple enough false positives, force enough human moderators to have to inspect the images, that the whole scheme becomes financially un-viable on Apple's end?
Of course, the only thing we could do here is slow this "progress" down. Maybe we can use that extra time to share the message that "this kind of technology is not ok" and that being naive about what this tech will be used for is almost guaranteed to kill more people than it saves.
But while so few people are thinking of it in these terms, or if everyone believes the attempt to be futile, it can't work. It's like anything - by the time people realise there's a problem, it's often too late to fix the problem.