Without a doubt cruising is not an "all-inclusive" holiday - and it isn't advertised that way anymore, but it was for many years and so the perception that it is all inclusive is still out there.
The regular cruiser community actually had a bit of a revolution in the early 2000 when the cruise lines started adding so many more and new "spending opportunities" onboard. I remember a LOT of people saying "I'll never pay to dine on a cruise ship" when alternative restaurants were added.
But, in fact, we give the industry credit for successfully portraying these onboard added costs as "optional" and not mandatory (and in fact they are optional in most cases).
Of course, we all consider the Internet mandatory these days, but a cruise ship is different. (well, even hotels still try to get $12/day out of you for Internet) but if you were isolated in the mountains your only Internet option would be a satellite uplink, and those are never cheap.
That is the way it is on cruise ships, but let's also be realistic. I would bet you most of the bandwidth onboard is used by the staff, not the guests. And those cruise ships could no more survive without the Internet for their internal systems than they could survive without fuel.
So, yes I do tend to agree that if nothing else the price of Internet should come down on cruise ships. River cruises actually offer it for free (but it is a terrestrial connection, not satellite, over the cell-phone network).
But cruising is a complicated product. It used to be more inclusive when there were less options; meals and shows were always included. Tours and alcohol have always been additional (except on riverboats where they are mostly included).
Every cruise is different - that's just the way it is.