just as other countries closed their borders to China it seems fair for China to now close its borders or not accept flights to/from other countries with uncontrolled spread of covid.
China does allow flights to and from the US on Chinese airlines - the "fairness" you allude to implies that US airlines should be able to fly to China as well.
>Beijing in March said domestic and foreign airlines could operate no more than one weekly flight between China and any given country
>The Department of Transportation said the March order had effectively banned US airlines
>In recent weeks, China has moved to loosen restrictions on charter flights from some countries but not the US
Even domestic chinese airlines are only allowed 1 flight per week between china and any country