It is petty when you consider that Hong Kong has extradition treaties with a bunch of other countries. Most countries around the world have extradition treaties. China doesn't want criminals from the mainland taking refuge in Hong Kong. Hong Kong is now part of China so I reckon China has a right to expect such a treaty.
There is another possibility that hasn't been mentioned much. Hong Kong could be used (is being used?) as a base for undercover operations for efforts to spy on and destabilise China. China doesn't need a foreign power on it's doorstep, especially one which has a terrible record in Asia and elsewhere.
