I think you're trying to apply rules designed to govern shipping in general to a very specialised and controlled situation.

They mentioned multiple times that because the stage is empty, its center of gravity is much lower than what you'd expect.

By whom? The drone ship has no on-board crew.

... which means there's nobody around to be injured or killed if the rocket falls post-landing. Maybe it's a precaution worth taking when they intend these stages for commercial re-use; for now, the whole thing is one big experiment.

They will of course have modelled the cargo scenario, and I suspect that Musk means that it's nice and stable when they do that -- but then shipping norms come into play because, like you say, unsecured cargo is unsecured cargo.

It needs to be both stable and secured, because only one is inviting woe.

By the time the rocket lands, there's very few fuel left, basically you see a near empty Pepsi can. It's as stable as it can get.

Just to elaborate: due to wave action and leverage on the underside of the hull, parts of the deck of a ship can accelerate downward faster than the acceleration from Earth's gravity, and while the deck underneath the rocket has fallen faster than the rocket and its heavy engines it can also lurch to a side. Even if the center of gravity makes it equivalent to a sheet of rolled steel, so that it wouldn't tip over, it could become displaced, then go overboard or cause unwanted damage.

While I suppose that the rocket could be heavy enough to lower the center of gravity of the entire vessel below the troughs of any waves enough to make it incredibly unlikely, it isn't just the prospect of tipping over that makes it important to secure cargo.

how much money and time would a robotic lashing system cost? hint: a shitload.

how much money would a few ditched rockets cost, in comparison? hint: less than a shitload

would a recovery crew approach this platform in rough seas, even if a robotic lash were in place? hint: nope

then what exactly is the point of securing this 'cargo'? hint: there isn't one.

problem solved.