It's odd, the rules photographers accept for themselves.
When I visit museums I see paintings mounted in all kinds of frames, from modern to rococo. One local museum has a Monet displayed in the most garish frame you can imagine - it resembles an ant farm. I doubt many people notice and fewer complain. It's probably the only framing material they had in that size, or they simply decided to go with a "period" style regardless of taste.
But photographs are nearly always mounted, matted and framed uniformly, like little obedient soldiers.
Why do we accept this? Is it because we believe it's right? Or because we believe it's what others believe is right?