To nitpick, Netflix wouldn't want to request letterboxed formats. They should want the native format, because I'm confident that every video player they support will do the letterboxing for them.
They definitely do not do a good job of checking what formats they're getting. If you watch Star Trek or The Twilight Zone on a 16x9 TV, you'll actually see bars on left and right because it's encoded for 16x9 even though the source is 4x3.
