I'm no expert, but this one sounds pretty basic.
I can't imagine how a US citizen, showing a US passport, could be denied entry into the US.
They might ask a question like, "What were you doing abroad?" If the person answers, "Serving time for SA," I don't know what they do, but I still doubt they can deny entry.
After all, we can't kick citizens out of the country when they are convicted.
Again, I'm no expert, though.
"This is not my shame, this is their shame." Mona Eltahawy