Well assuming that the original supplier of the internet has full capability to shut off all connections to his or her internet for the people without a password then no, it is not stealing. Many people share their internet much like people share a bench. Does it benefit anyone to not allow someone to sit down? Does it hurt the person to let someone sit down? No. So why is internet sharing different, it doesn't hurt the person that owns the internet connection, nor does it help them in anyway. It is not stealing and ethically it is correct.