Why do non-Southerners keep insisting on this? Bless your heart can be said sincerely or ironically, like pretty much any other phrase.
The ironic usage makes for compelling dialogue and comports with stereotypes about Southerners as formal/restrained. So that's what ends up on television. At least that is how I think I came about having that impression.
You mean kinds like "I wish them well" here?
My comment is an internet comment about idioms, not a comprehensive linguistic treatise.
You seem like you're looking for something to be upset about. I wish you well.