You're not missing anything. There is no interface contract regarding what container a component can be inside. If you want to restrict what kind of components can be added to the container you can overload the Add method and do a check of the type of component being added:
public class Entity : Container {
public string Foo = "Bar";
public virtual void Add(IComponent component) {
if (!typeof(Position).IsAssignableFrom(component.GetType())) {
throw new ArgumentException(...);
}
base.Add(component);
}
}