Firing on and sinking a flagged vessel and possibly killing crew (also, how do you then prove that you DIDN'T kill anyone?) is exactly the kind of thing that gets governments openly involved. Seizing a private vessel and fining the owners? That's a civil matter and while there may be backroom discussions everything stays civil.

"The military of Country X is firing on and sinking our country's vessels" is much more likely to lead to retaliation. China may not have the navy or willingness to project power across the Pacific, but I guarantee they have the capability if focused to utterly destroy the computing infrastructure of the country - and while enthusiastic private parties may not get involved in civil penalties, "Argentina just declared war on us!" is a hell of a rallying cry.

Edit: also keep in mind that even governments are not monolithic entities with a single mind. Those costs are coming out of someone's budget.