Look at JZ'mann year this year, and you understand why they did it. True Straus ws having problems, but his last three games have been beauties. He just needed to settle down. Z'mann is now 8-2 with a ~1.70 ERA; Strasburg has a 2.49 ERA and has pitched 23 innings in his last three consecutive starts, is averaging over 100 pitches a start and has thrown as many pitches this year as Justrin Verlander. (Before this, Strasburg had never had more than 20 innings in a span of 3 starts).
Considering that these guys should be anchoring the rotation here for years, I think Rizzo was justified and has been proven right in doing what they did.