My rebuttal is that an account ID picture is not necessarily a portrait photograph. Oh, sure, social data miners like Facebook want you to only use a photo of yourself for a profile picture. But many people -- such as on Twitter -- don't.
Profile pictures I've used are a photo of my computer, a still from a movie, an animated GIF of a flying pig (alas no one I know of supports animated profile pictures), and yes even a photo of myself every once in a while.
That said, complaining about the shape of the profile picture is pure bikeshedding. Big deal. How much time on your computer do you spend staring at your own picture? (Please don't answer that.) Although if given the choice I'd prefer rectangular. My computer screen is a rectangle. The area reserved to display the picture is a rectangle. Does a click in the invisible rectangle register as part of the picture? And most of all, it's easier to crop a photo to appear circular when displayed in a rectangle than it is to show a rectangle inside a circle. So the "safe" default should be a rectangle.