Interesting question, one I have also asked.
I have been told that the original cars that left AC Cars had proper British leather seats. But because the cars sat out in the open unsold for so many months, the leather seats became weathered.
When Shelby American's East Coast sales manager suggested to Shelby that they could be converted to and sold as S/C's, it was determined that the seats needed to be recovered, and the choice was the less expensive vinyl.
Anyone want to jump into this one?
Bud