there are many more that do not fit that classification. On many of these, the stem is a bit darker than the rest of the rock, though still lighter than the sphere. Does anyone have a color version for this image set?
Last chance long horizontal stem sphere

In this next image, the darker color of the short, often curved stems is visible. Both the darker color, and the curvature argue against the stem being a hoodoo. You guys points about wind direction are right on, that Mars can have hoodoos in all directions, but many rocks like this next one, and Stone Mountain too, showed darker stem like features attached to spheres in many directions.
(I can't find the right image, but this is similar)
dark curved stems