I think it's a bug.
A lot of the built-in editing features seem to save changes as metadata, i.e. they don't actually change the picture - they just store a little data with it, e.g. "Brighten by 20%" or "Rotate 90-deg CW" or "Crop from X,Y to X2,Y2". Sometimes that metadata isn't recognized by other apps or services, e.g. I tried cropping a picture with the built-in editor and then uploaded it to Facebook - the uncropped picture showed up.
Most of the third-party editing apps (I use PhotoShop Express a lot) actually save a copy of the edited photo as a new picture, and Photo Stream picks those up properly.