I don't think there's anything wrong with leaving constructive criticism on an anonymous gift fic - but the comments I think are being referenced here didn't really strike me as all that constructive. It seemed they were more like complaints about personal preferences. Not to be a total bitch, but why should anyone care if someone didn't like the pairing/dynamic the giftee wanted? We already know it's not their request so that sort of criticism is just irrelevant. Whining on about it in a comment is pretty wanky behavior, and you're absolutely right to be offended by it.