Hi zbox
If G scrambles, that creates a naval battle. All units must participate in the naval battle. So, can’t bombard. No scramble there is no naval battle, so can bombard.
Note
In a situation with a just a sub or a trprt, those could be ignored, no naval battle and you can bombard.
If you wanted to sink them though, that creates a naval battle and no bombard.
Panther will probably explain it a little more precisely :)