That's not a requirement. They could have gone after LSU- Miss St if it was (which judging by the time slot, was probably ESPN's 1st pick).
CBS's SEC TV contract rights are a bit weird because they were written based around when CBS still held the rights to show the U.S. Open (which always goes into the second weekend of college football). Their "first choice of SEC game" doesn't contractually begin until the third weekend of games.
This year - for the first time in a long time - CBS isn't showing the Open (ESPN bought those TV rights) and has those first two weekends of the season open...but their contract with the conference still says that their first choice doesn't start until the 3rd weekend of the season.
I think - in order to show something - they had to more or less "buy" two SEC games from ESPN.