It's not the waters that's important here. It's the debris passing through the flight path.

Qantas doesn't own the flight path either?

Oh common?! Do you own the travel path when driving down the highway? No you don't but there are agreed upon and codified rules on right of way that protect your right to safe passage or navigation. Similar convention applies to air space and air travel, look up Annex 2: Rules of the Air by ICAO which outlines right of way principles for air travel

What does it say has right of way between a rocket stage, that cannot be steered, and aircraft, that can?

> between a rocket stage, that cannot be steered

This is just trash. Debris as they call it.

If drive in front of you on a highway and throw some "debris" out of the window, will you enjoy it ?

Will I enjoy it? What's that got to do with anything?

The FAA confirmed that debris fell outside designated areas, temporarily disrupting air traffic and causing several aircraft to divert or delay flights.

You can read more here and other news sources

https://www.foxbusiness.com/technology/spacexs-starship-grou...

Another way of phrasing the situation is that Quantas _very inconveniently_ chose to put their flight path straight through the projected trajectory of rocket debris.

Would you ask the same question if it were Long March rocket debris falling into the Atlantic with very short notices from China?