That makes sense. Quoting from the data model: "When inheriting from a class without __slots__, the __dict__ attribute of that class will always be accessible, so a __slots__ definition in the subclass is meaningless".
In the current case, for the little-endian systems, which I presume you have, BigEndianStructure is a subclass of Structure, but it doesn't have a definition of __slots__ in the body, leading to your results, according to the specificaton from the data model. In this case, it makes sense for Point1 to have __slots__, but not for Point2.