The problem is one of capabilities; unrestricted image hosts delegate too much power to uploaders. An uploader should not be able to create a valid URL with any data under the hoster's domain.
In addition to what's been mentioned, another possibility is to force users to bring their own domains to the table when doing hosting; the user is free to host things from the service at their own domain, and the hoster's domains remain distinct and untainted.
In addition to what's been mentioned, another possibility is to force users to bring their own domains to the table when doing hosting; the user is free to host things from the service at their own domain, and the hoster's domains remain distinct and untainted.