I blame that on the director. In the book, it was clear that the reason she didn't want to participate is because after her entire district was bombed, she concluded that anything she does against the Capitol ends up with more people dying on her shoulders, just like in District 8. Peeta was one of the main ones she felt sorry for, though. I guess the script writers focused more on the Peeta aspect. Didn't like how they did the D8 scene either, the camera angle made it look like she crashed the plane into the hospital, plus in the book, she shot rounds of planes, not just one. Oh well.