I have got a ONE entry VISA from SHANGHAI EXIT ENTRY BUREAU why do i still need a return ticket ?

i might not make u understand last time..i wanted to say I HAVE GOT A ONE ENTRY VISA FROM SHANGHAI EXIT ENTRY BUREAU so that i can go to PAKISTAN and come back to SHANGHAI again in 2 months.they have allowed me to come back to SHANGHAI....i mean i have that BLUE VISA L type from SHANGHAI office then why do i still need to buy a RETURN TICKET from PAKISTAN to go back to PAKISTAN when i want to stay in SHANGHAI?????? or can u suggest me any other thing so that i should not buy a return ticket from PAKISTAN????? can i get any other VISA.? PLZZ HELP