Exterminate all rational thought
What?Don't know about Publix, but if the butcher at the H.E.B. said that to me, I'd look around to see who else he was talkin' to.
The second quote, well... the Southerner might be talkin' to a single fan, but the reference is to all misguided fans of midwest teams. That Southerner was politely acknowledging that while the person he was talkin' to might be deluded, it's a common delusion and nothing to feel too terribly ashamed about.
For the first, this is a fairly common usage, especially when dealing with differing levels of authority (formality). So while it is often confused with the "missing plural," it is missing formality.
For the second, it is used when a Southerner is talking to a non-Southerner, as a marker. For example, I would say the following to you-
"Y'all don't know jack; Texas A&M isn't a real SEC team, and Texans can't play football. S...E...C!"