This was a smart move for a sometimes stubborn company and, as an early F30 owner, I saw this coming a mile away.
As others have pointed out, there's nothing wrong with stop/start in theory, but the F30 application as a starting point for the NA market is highly questionable. For one thing, the 3 Series is the soul of the BMW brand -- not the car you want to monkey with when you're trying out new technology. And the feeling of stop/start with the N20 really cheapens the car IMO. My co-worker drives a last generation Camry Hybrid and that car hides the impact of its stop/start feature far better than my wife's 328i.
To me, stop/start is like ABS, traction control and electric power steering -- all were valuable additions, but nothing this impactful to the driving experience should be added to a car as a standard feature and forced to be used by default before it's proven under all conditions and it's been run through a real world customer acceptance process (focus groups, etc.). This is doubly true for a premium brand like BMW with a performance pedigree. Get it right, or don't implement it right now.
In short, I think BMW rushed the intro of the stop/start feature in the NA market without working through many customer-facing implementation details and I think they're hearing significant negative feedback about the decision, hence this change. And I can guarantee you that there are relieved BMW salespeople now telling customers who are annoyed by stop/start on their test drives, "don't worry -- the service department can disable that for you."