Description:
When I click in the menu entry of java web start it starts the cache viewer (by calling javaws -viewer)

but it does the same when I click (the desktop does not matter) on a .jnlp file instead of launching the file with javaws. One has to use the CLI to launch a .jnlp

I suggest to use a wrapper to fix that bug.
Change java-web-start.desktop file to use that wrapper i.e.
Exec=/opt/java/jre/bin/javaws-launcher %f

and the wrapper javaws-launcher being:

#!/bin/bash
if [ -z "$@" ]; then
exec javaws -viewer
else
exec javaws "$@"
fi