I'd argue it's rather healthy for americans to not just assume that someone spouting rhetoric is being sarcastic. That attitude of extending the benefit of the doubt to people undeserving of it got us into this mess in the first place.
(Tho in this case, it's not that people don't realize they're being sarcastic - it's that what they're saying with that sarcasm is really, really off the mark, and they're being incredibly shitty in how they said it)