No it's not a bug, they always do that, very annoying. If you want to make a vehicle that will never move you can disguise a gate as a vehicle.
To do that just go to gameplay objects, choose a gate and then under the options for the gate choose disguise, the sporepedia wll pop up, then all you have to d is choose the vehicle
You have to be careful not to have a key of the same colour nearby, or likley to come near it though or both the key and the prop will dissapear! It also isnt any good if you want it to attack or move later on.
I hope that helps a bit?