For comparison, it's pretty obvious that there are certain things in physical realm that citizens shouldn't have. So it would be wrong to claim "my things are mine and no one should be able to inspect them or take away from me, no matter what those things are". Examples for things citizens should not posses: weapons of mass destruction and slaves.
Apple et al argue that CSAM should be digital content that nobody ought to possess. So while you're obviously technically capable of storing it, it shouldn't be legal.