I have mixed feelings about that. On the one hand, you're right that people should be made distinctly aware when something is being censored. On the other hand, you have to be careful not to convey the sense that the user is in the wrong or frighten anyone into self-censorship by giving the impression that the user could be sold out to an oppressive regime by the service provider.

There's no reason a site couldn't serve content along with the 451 code, just as HTTP200 is served with content.

In fact, that's a SHOULD in the RFC:

    Responses using this status code SHOULD include an explanation, in
   the response body, of the details of the legal demand: the party
   making it, the applicable legislation or regulation, and what classes
   of person and resource it applies to.

Or how HTTP 404 is served with content.... (e.g. http://github.com/xyzzyeieio - there's a good 134KB text/html with your 404 error code)