I can delete photos. Just give me the url of the photo you need killed and I'll happily delete it for you. But also, don't (accidentally) upload a nude.
Assuming it's not a joke that flew over my head, how could any individual instance remove the images once replicated? Is the removal from the original instance cascaded?
That's not true, images are also copied over. This is also for efficiency reasons and to spread the load of the image out to the servers. Sometimes you'll see images not being copied to your own instance, but that might be because your instance has a lower image size limit than the instance it was uploaded to originally.
There's no technical reason you can't delete an image that's been replicated. There's an API to replicate the data, there can also be apis to delete the replicas (and apparently there are?)