Since Apple shared the algo, people who trade in these things already know how to get around it (just adjust the image so the fingerprint changes). People who trade in these things will probably turn off icloud too.
Similarly, governments could take a known image they want banned (say of a national embarrassment), grab some CSAM, tweak the signature on the CSAM to match the photo they want banned, and add the tweaked photo to the CSAM DB.
i.e. my iphone told me this photo was in the database, so I'll start adjusting it until it 'passes', and them I'm free to share it without detection.