It's a Southern thing
ya'll [sic] wouldn't understand
Now, I'm not going to call out a stranger in public, but just for the record, I'd like to clarify a little. It would be foolish of me to speculate on the motivations of that particular man in selecting that particular t-shirt. Regardless of his intentions, that shirt is wrong on so many levels.
The adoption of this flag as an emblem of Southern Pride is directly tied to the struggle for civil rights: it was first flown over the South Carolina capital in 1962, which is not the 100th anniversary of the Civil War (1861-1865). Whatever its adherents claim, the flag is a smack in the face to those who fought against slavery and for civil rights. That flag invokes white supremacy just as the Nazi flag invokes aryan supremacy. For more, visit Same as It Ever Was: South Carolina and It's [sic] Flag.
Sadly, racism is not exclusively Southern. In fact, by some measures, incidence of sundown towns, or housing segregation, the South is arguably the least racist region.
And illiteracy isn't a Southern thing necessarily, either. Many Southerners know that the correct spelling of the abbreviation for "you all" is y'all.