it's not that they can't really enforce it, it's that they don't want to. disallowing the unauthorized posting of copyrighted images would destroy any social media platform; it's the main thing they're used for.
in general, posting copyrighted images is not in itself something a platform like cohost can treat as an ethical problem. they can and should treat fraud as banworthy, whether that's passing work off as your own when it isn't, or trying to make money by selling someone else's work (without sufficient transformation) without their permission. but just "stealing" Garfield strips is never going to be actionable.
of course there's a complex line between mass-market stuff like garfield and, say, fanart by an indie artist trying to build a reputation. you can see in the "behaviors to avoid" section of the current community guidelines that cohost is drawing that line loosely at "posting people’s content without attribution" plus "if the creator asks you to remove your post, you should listen to them." this seems fairly reasonable to me.
so the sale of generated images is pretty ethically dubious, for the reasons you mention. (even that is complex, because they clearly are transformed, and you don't want to litigate it in a way that makes selling collages illegal. but it's also pretty clearly on the other side of a line, and i certainly hope we end up with laws that protect artists.)
but selling images (or using them in your for-profit videogame, etc) is very different from posting them on social media, the same way that posting a Garfield strip is different from selling bootleg collections of Garfield strips on Amazon. of course there's a new problem, which is that the creator can't meaningfully invoke the "asks you to remove your post" clause, because they can't know or prove that your generated image has their work in it.
i guess in typing this my opinion has evolved a bit, in that i do think that fact is conceivably consistent grounds for a more wide-reaching ban. but i still don't think it would be good policy, or that they'll try it, for several reasons:
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many, many users simply do not see generated images as theft, and will feel personally insulted and angry if they are told that posting them is unethical. mandating that they be tagged is honestly an admirably aggressive stance in my view.
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the harm to the creators whose work has been stolen is impossible to quantify, in a way that categorically distinguishes it from the "reposting fanart" case. there's just really no case to be made that you are harming someone's ability to grow their own audience by posting generated images that may have had their work as one of millions of inputs.
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not all generated images are theft; there are models that only include work in the public domain or by people who have consented. it is impossible to prove (or in many cases even for the original poster to be aware) whether or not a given image was generated in this way.