It's not to do with RAM. It's because Construct 2 doesn't know what to set for the object's properties if none exists. For example, the instance variables, object settings, behavior parameters and so on are all empty. So if it had a bullet behavior, what speed do you use? Zero? The default value? I don't think either is what you really intend.
We could fix it by allowing you to set properties when no instances are selected but for architectural reasons it's a little tricky. The workaround is to use an unused layout where you dump your one instance of an object.