Hi Marco - a solid can only have control points turned on if each surface in the solid are "naturally trimmed", which means the trimming boundary corresponds to the boundary of the "underlying surface", and each of those underlying surfaces has the same control point structure at their common edges.

That's the case for a box and also things constructed from say sweep or loft (unless capped), but if you take a cylinder for example and use Edit > Separate on it and then turn on control points for all the individual surfaces you'll see the caps on the cylinder have an "underlying surface" that is a 4 point plane and the circular shape of the cap comes from a trim curve that is internal to the surface.

It would not be good to be able to turn on control points for solids that are like that because moving one of the plane's corner points for example would then open up a gap in the solid where the cap and cylinder would not meet up anymore.

This is illustrated and explained in more detail in this FAQ answer:
http://moi3d.com/wiki/FAQ#Q:_Why_does_show_points_work_for_some_objects_but_not_others.3F

- Michael