It's not space debris, it's the deliberate disposal of the upper stage of the rocket precisely to prevent it from becoming space debris. The time and location of re-entry are planned and controlled. This is not going to crash into your neighborhood (except if you're neighborhood is in certain areas of China, where they they happily dump spent rocket stages on populated areas).
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.
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
The FAA confirmed that debris fell outside designated areas, temporarily disrupting air traffic and causing several aircraft to divert or delay flights.