I am not Omid, but: It feels like an attempt to sneak something past the reader -- and indeed that's clearly what it is. The writer might defend it along the following lines: "Few readers would listen if simply told to do Y, because it's very different from what we're used to doing. But, in fact, principles we all accept mean that we should all be doing Y. So I need a way of explaining this that won't get blocked off at the outset by readers' immediate aversion to the idea of Y." Which is fair enough, but if you notice it being done to you you might feel patronized or manipulated. And while we might like to think of ourselves as making moral judgements on the basis of coherent principles, it's at least arguable that really those principles are often less fundamental than the judgements they purport to explain -- so the reader's actual conclusion might be "... so I need to revise my moral principles somehow" rather than "so I need to do Y", but the argument is phrased in a way that rather bypasses that conclusion.

Having said all which, I'll add that I don't think I myself find that sort of thing obnoxious; while I can't think of any case in which I've done the same myself, I can imagine doing and don't feel any guilt at the thought; and I think that even if our moral principles are derived from reflection on more specific moral judgements rather than vice versa, it's reasonable to give precedence to such a principle even when a specific judgement turns out to conflict with it. So I don't altogether agree with the argument I'm putting in Omid's mouth.

(Which is probably reason to doubt whether I've correctly divined his meaning. Omid, please correct me if my guess about what you find obnoxious is wrong.)