It's not as simple as "just don't switch Kemba onto Lowry".
There's a reason ball screen offense has become the norm for almost every team: no matter what you do, you give something up. If you don't switch the screen, you either risk a foul by fighting through or you risk a three by going under (and Lowry was 6/10 on those tonight). Worst case, you give up a three AND foul when you run up the ball-handler's back. You could blitz him, but Lowry has good vision and will likely pass out of it... now you have three to guard four. There's no perfect answer.
Kemba has to handle his business on defense, and in the playoffs, he generally has. And on that particular play, I don't really fault Kemba. Lowry hit a tough shot. That happens.