Here's the thing, your actions are different in either case. If you don't collapse the group (not necessary), you can see it clearly.
Clicking an event (or group) selects the event and its sub events.
Dragging an event will leave only the dragged event selected.
This doesn't have anything to do with groups in particular, nor top level or sub level events. It will happen if you drag anywhere, from top level to sub or vice versa, and it happens with sub events as well.
If you drag a group to the top level, then click it, it will select all sub events as well.