Allow me to be pedantic right back. I did not say "sarcasm is". I said "we call that sarcasm". It's not really to label a description of how a word is used as "incorrect" and use a proscription as evidence. My fourth sentence was a bit regrettably absolutely phrased but the context of my statement should be set by then.
I don't even care if that's true of all of us Americans, it's true of enough. (The idea that Americans don't get "sarcasm" is very foreign to me; oh, I certainly know people who don't get it but we seem a fairly sarcastic culture to me.)
Okay. I think those sources are descriptive, but to be sure, here is the American Heritage Dictionary (http://dictionary.reference.com/browse/irony), an explicitly descriptive dictionary about American usage. It seems to agree with the other sources that irony can be and is often used in this case.
On the other hand, one can reasonably believe that every dictionary is, by its nature as a reference book, primarily prescriptive (and proscriptive) and therefore cannot be reliably descriptive. If so, then it seems we do have to go only with personal observations.
In terms of personal observations, I agree with you that many Americans I know would use "sarcasm" in this case and not use "irony." I think I (along with many other Americans, maybe fewer than the other group?) would use "irony" and probably not use "sarcasm" because it doesn't have that particular, hard-to-miss sarcastic tone of voice.
I don't even care if that's true of all of us Americans, it's true of enough. (The idea that Americans don't get "sarcasm" is very foreign to me; oh, I certainly know people who don't get it but we seem a fairly sarcastic culture to me.)