The reason you can select the dimension is so that you can edit the associated relationship. It's sort of a shortcut so you don't have to go look through the dropdown list on the command bar. You can turn off this behavior by clearing the 'Show all dimensions' setting on the Assemble Options dialog:
Thanks Greg, that does help a little. What you are saying is desired behavior and I find that feature handy when evaluating the assembly. I do not find that feature handy while in the place relation command. My brain does not find it logical to want to edit one dimension while trying to select an element in the relation command.
Sometimes the dimensions get in the way, using a 3D input device and sluggish graphics I wind up clicking a dim instead of the element I want. Then I'm in an unknown state, am I still in the place relation command? am I editing a dimension?
Those who know me question the assigned user rank.