Where was I wrong?
Easy, you are assuming it's his network or a free public network. However "tap into" typically means you are not supposed to go there, kinda like tapping into your girlfriends bank acocunt. It's the same kind of concept. Had he said "log onto" it would be totally different.
I'm required to treat this like I would torrents. It may just be legal, but I can't allow it on the chance that it isn't. So drop it.