This shouldn't be the case in a society that supposedly values liberty.
The reason why the Supreme Court upheld bans on possessing CSAM is not because it's obscene, but because it incentivizes abuse of children to produce it.
We also acknowledge that participating in pornography requires consent, and that the continued distribution of nonconsensual pornography constitutes a continuing crime against the unwilling subject. Because children have zero legal capacity to consent, CSAM is de facto illegal.
It misses the point to think that CSAM is illegal because it is "obscene". It isn't illegal because it's disgusting; it's illegal because it's egregiously harmful to children. It's like thinking the bad thing a murderer did was make a mess.