Ariana Grande is a cute little airhead who was filmed last summer licking donuts, putting them back on the shelf and declaring "I hate America." She apologized a day later, and apparently that made everything OK with her fans.
Her concert in Manchester, England was the perfect venue for the latest ISIS- inspired suicide bomber. The demographic of her fan base is heavily weighted toward pre-teen girls, kids who have little or no grasp on politics outside of who's most popular at their middle school.
If you're intent is to create terror, what better group to attack?