I would say the difference between drugs and images is that the first amendment doesn’t protect drugs. This then becomes an argument about the limits of the first amendment.
The US constitution doesn't constrain French criminal law. But regardless, you're wrong in the US too: the first amendment absolutely does not "protect images", c.f. the fact that you can be prosecuted for distributing CSAM at all. We've already crossed the rubicon where we agree that the "speech" is illegal. The quibble under discussion here is whether or not the justification for the ban applies to synthetic images.
To be pedantic, that is only true until/if the Supreme Court rules that it is a violation of the first amendment. That is to say, it is simply the status quo and not a hard fact that the first amendment doesn't cover images and text/speech equivalently.
