Hi Happy2010,
I have put your comments in quotations and answered them. Hopefully there are cleared.
"On one hand, if a F1 visa holder can not work with an EAD card granted while his AOS is pending, so why the card was approved and sended ?"
The moment you file for AOS, and the application is pending, by USCIS standard you are legal but not F1. So yes, you can use EAD to work even if you didn't have your interview but have the EAD.
"on the other hand, as the AOS petition is pending, the petitioner is still on the F1 visa status, therefore , as from his so-called F1 visa status, he is not allowed to work.(except from on campus)."
If AOS is pending, then you are not on F1. But if you have EAD, then working on-campus or off-campus both are legal. The only difference is you might not have to use the EAD for on campus, as you were already authorized by INS to work on campus as F1. But when you want to work off-campus, then all the paper work would have to be done based on the EAD.
Now people usually have the concern if the application for some reason is denied what happens. I read somewhere that in the time when the application is still pending, if you stay on F1 rules (i.e. take full load of classes and work on-campus for up to 20 hrs a week) you do not invalidate your F1 status and would go back to it. The only concern would be if you traveled outside US as the IO at the entry port could refuse entry on F1 as you had shown intent to stay. But if you stay in the US, it shouldn't cause any problems!