Because of the way ABS plastic injection molding works, it is important that parts have a uniform thickness throughout, and don't just have a large section completely filled with solid plastic. This page explains a little about that:
As the plastic solidifies in the mold, it freezes from the outside (near the mold surface) toward the inside. In thick sections, this results in inward pulling stresses (due to contraction) that can cause sink marks in the outer surfaces of the part. In addition, because thinner sections will freeze faster than thicker sections, there is also the possibility of stresses building up between thick and thin sections, resulting in part warpage.
The reason that the stud is open is because that area of the part couldn't be reached from the bottom as usual due to the closed slope underneath. The constraint of uniform thickness forces either the top or bottom of a stud to be open. This is the same basic reason that the studs on Technic beams are open as well.