If there were backs on the bleachers, there would be even more injuries from people climbing/falling over them than there are now with backless seats. (Even with telling people not to climb over the rows, it still happens, and people still trip and fall flat on their faces) If you'd prefer a seat back, bring along a collapsible seat designed for stadium bleachers.
As for the stadium vs Disneyland's setup, while it loses a bit of the charm of the middle-of-the-park location, in my opinion the better crowd control in the theater and the park itself much outweighs the lessened atmosphere.
EDIT: Just wanted to add one more thought: I'm surprised the Merchandise people haven't taken the initiative and started offering Disney-themed collapsible stadium seats for sale in and near the amphitheater...
-Rob