Man on 1st with 1 out becomes RISP with 1 out. That is a flat-out worse situation. Take a look at the run probabilities. If it were really as clear-cut as you make it sound, Burnett would IBB whenever there was a runner on first, but he doesn't. It's all about who Wright is, and he is not enough of a threat to make it worth IBBing him. Besides that, Burnett is not just a GB guy, this season he is also a strikeout guy. By IBBing someone you completely eliminate the potential for the K, which is obviously the best outcome.
It's a good decision because Wright is a 1.062 OPS hitter vs. Lefties. He has always crushed them in his career.