If I reject a file while in the viewer it gets dimmed and a red X.
If I, however, display a file in the viewer which already has a reject rating it gets displayed normally.
I think it should also be displayed the same as if had been rejected from within the viewer.
Any arguments against this?
No. There are files with a reject rating. And you can "mark files for deletion" (or another option to choose from) while being in the Viewer. You mark the files, then decide what to do with these files after leaving the Viewer.
Very different concepts.