The ‘sweet tea only’ thing was true 20 years ago, but more recently, in both Carolinas, I have been asked if I wanted my tea sweetened or unsweetened.
I agree; it’s not uncommon to have unsweet available in restaurants, but sweet is definitely consumed quite a lot more.
The reason I responded to your post is that I was outside Charleston not long ago and I ordered a glass of sweet tea at a seafood place. They told me they didn’t have sweet tea, and I was utterly stunned. Close to speechless. They explained that they get a lot of Yankee tourists and just don’t sell that much of it, but it just wasn’t a good meal without sweet tea. I wouldn’t go back.
As an aside, Chick-fil-a, hands down, makes the best fast food restaurant sweet tea. It is ALWAYS good, every time.
Back in the 1970’s, while living in Chapel Hill, NC, we used to debate the exact location of the “sweet tea line,” above which the default in a restaurant was unsweetened tea and below which it was sweet tea. I recall we finally decided that the line was approximately coincident with I-40/85 as it ran east-west between Winston-Salem and Durham.