Hello Andrew,
Most diagrams are using the same functionality – this is the default behavior in the true diagramming packages like Visio and others.
Nevron Diagram is not a tool for drawing rectangles but a complex diagramming framework for schematics and various types of diagrams. Maintaining the aspect ratio when using the diagonal handles is natural for diagram shapes (imagine resizing a monitor shape which has 16:9 aspect). However, if you would like to have this behavior and change the aspect ratio, you could simply use the Create Rectangle tool or to use the Shift key with the Rectangle handlers.
For the current version, we can override the default behavior for you as part of the Nevron Custom Development services.
We can also consider adding this as an option in future releases of the Diagram.