As to whether geometry is selectable for WAVE linking or not depends on the preference you (or your company administrator) has set in Customer Defaults. In Assemblies-General-Product Interface you can choose Interpart Linking allow method. Either any geometry, or just product interface geometry or warning if user selects geometry that is not a product interface.
It can be useful for the author of the data to control what other people can link to. This reduces the chance of the wrong geometry being selected.