Because it's unnecessarily tactless and bordering on hurtful. Learning how to give honest feedback without being so cutting is a skill the commentor should learn.
How do you define degree of tactlessness? It was so nice of him to call that website 'ugly'. It is totally unacceptable to downvote someone for telling the truth(no matter how ugly it is).
Exactly. There's this poisonous atmosphere in HN that dislikes criticizing honestly. In fact, throw in some compliments and you'll get a zillion upvotes. But complain about something, and fear getting banned...