Why can I only select node points with code values > 0 in Data Viewer? 

The Data Viewer contains the option to filter the active objects (nodes or elements) before you use the Node selection polygon area to select values. In general, this filter functionality is not active so all elements or nodes can be selected.  If you select the item ‘Code values’ in a mesh file, the default setting is to select only nodes with code values > 0. 

You can modify the filter from the Item Selection Filters dialogue in the Data drop-down menu. For each item you can activate or deactivate the filter entirely, and/or specify the filter settings by an operator and a value.
To be able to select node points with any code value, uncheck the box for ‘Code value’ to render the default filter inactive - see example in Fig. 1


Fig. 1 - Example of Item Selection Filter dialogue when viewing .mesh file