This is just legal cover until such time as its possible to enforce no child exploitation imagery, no copyright stuff, etc. It doesn't matter if they are able to enforce it, Valve can use this policy as cover if they ever get sued. Don't overthink the motivation. They will not even have a bulletproof way to detect AI imagery as it evolves every single day as an arm's race and detection is a full-time job. Even a FAAN…
If the art used in a game violates copyright or contains imagery of exploited children, ban it of course, but what does that have to do with whether it was generated via AI or created in another manner?
If anything AI generated art should be _less_ susceptible to copyrighted stuff because everything is original (even if it's not in original style)