As I understand it there is no mistake from Ben Burtt's part : reversing the rear channels was intentional. Don't ask me why he did this though, sounds as dumb to me as it does to you.
I think it's more likely that its was a total cock-up and the whole "creative decision" spin was just so they didn't have to replace all the discs sold with new ones with corrected audio. If this was a creative decision then why are some of the front channel FX reversed too?