I call this a bug because it inhibits some of the creative uses armor stands provide. Simply put, when you put an armor stand on top of a mob using the Passengers tag, the armor stand is given control of that mob because the rule of passengers states that the topmost rider has control of the mount(s). However, this doesn't make sense that an armor_stand, an inanimate entity, should prevent a moving entity underneath it from freely moving around because it has "control" over it.
If I wanted to use an invisible armor stand to display a customized model or display, I would be currently limited to keeping it immobile unless I hassled to sandwich it between a mount and a second rider to make the whole thing mobile.