While there is no doubt that the "common meaning" now is just anything that's very easy, among the locals I grew up with the term "shooting fish in a barrel" nearly always implied a strong condemnation of "unsportsmanlike" conduct of some sort. Anything compared to "shooting fish in a barrel" was likely something it was understood that "you just don't do."

I suspect that that meaning has pretty much faded out of use - the modern generation seems to like everything as easy as possible...(?)
but it might be of help if someone really wants to look into origins of the term. Comparable, but less used, terms were "hunting chickens/duck/birds in a pen," "shooting pigs in the barnyard" and/or "fishing with dynamite."

The usual connotation of any of these terms was that only an "incompetent" would take pleasure from any such "contrived" situation. There apparently is no shortage of such "incompetents," since the major game law convictions in my area recently have been for things - all illegal here - like "herding" game to the "hunters(?)" using aircraft, 4WD, or ATV vehicles, using radio/cb/cellphone communications while hunting, and/or "shining" - using spotlights (or headlights) to blind the game at night. All much like "shooting fish in a barrel" - in the older sense as I learned it.

John