Regardless of whether there were other empty seats or not, anyone can choose to sit anywhere they like in the departure gate (other than those reserved for handicapped and the elderly.) But, I wouldn't sit on someone else's luggage, laptop or not, without asking them to remove them first.
I kept my sad and departed expensive laptop half way under the seat (so that I could keep an eye on it and it didn't disappear from the back!) but the person seated next to me suddenly got up and stepped on the laptop.