I dont think (in fact I am pretty sure) that this forum rule has nothing to do wiht laws.. more with common decency to fellow photogrpahers who arn't here to defend any criticism..
i've seen that being as the reason for this policy stated on this forum before, but it is logically flawed, surely?
for instance, there are plenty of outlets for people to critique things where the original 'artist' has no means to defend or respond to such criticism - newspaper reviews of films, tripadvisor, magazine reviews, amazon reviews and so on.
nobody says "you can't write a bad review about a film because the director isn't given an opportunity to defend himself", or that you "can't write a bad review on tripadvisor because the manager can't respond or that it might damage his business or reputation".
are we saying that you can't have an opinion on a photograph unless the photographer can be there to rebut anything negative that comes their way?
it's the glaring unbalanced inconsistency which seems strange to me and it seems strange that photographers are given this special privilege when pretty much everybody else in a creative industry isn't by wider society.
i look at it this way: if you don't want people to say unkind things about your work, don't put it in the public domain. i'm not saying it would be very nice to be on the receiving end of a review that concludes something that you are very proud of or have invested a lot of time into is a load of old crud, it will surely hurt.
but i also like to give human beings the benefit of the doubt: just because somebody says a photograph, film or computer game is sɦite doesn't mean that it is, i'm clever enough to make up my own mind and i think most other people are too.