To use the word "worst" is to take an extreme slant and reveals a sensationalist bias on the part of the user - which is to be expected if the spin was induced by a newspaper or other media outlet.
There is only 15% between the bottom and top ratings in this survey. Somebody had to have the highest score and somebody the lowest.
This survey does not mean that Qantas service is bad. The survey only made it into the news because the particular newspaper was able to come up with an angle that was deemed sufficient to excite readers.
Qantas has been doing it tough in recent years so one should have been expecting precisely this result.