It's a private nonprofit that appears to be wholly funded by the government. It's reasonable to expect that if they don't "play nice" with the DOJ, their funding could be affected. Incentives are important.

https://en.m.wikipedia.org/wiki/National_Center_for_Missing_...

But it’s not something that the NCMEC can do in secret. Apple gets the CSAM reports and decides whether something is worth sending as a tip to NCMEC.

I wouldn’t think the government shares the images with NCMEC, wouldn’t they just say “we captured some hard drives, add these hashes to the list”?

My understanding is that the NCMEC actually has the pictures, and they’re basically the only people in the US who can legally hold onto them.