As others pointed above, nothing prevents potential illegal resharers from using a separate bittorrent client.
Yes, and as I explained before, that is completely irrelevant point to what I was actually saying.
That argument is similar as saying that GOG could just as well share all GOG games as torrents to everyone (instead of restricting downloads only to those who have paid for the games), as anyone could pirate all GOG games from elsewhere anyway, if they really wanted.
Humble Bundle provide an option to download their games through bittorrent for those who bought them, since torrents provide a flexible downloading / resuming / repair out of the box. Anyone can probably pirate those if they want, so this kind of access is pretty loose since regular bittorrent protocol doesn't have an authorization layer. Anyway it's not the same as worrying about users uploading those games to bittorrent networks. So it's exactly like you said - HB know that pirating already happens anyway, so they decided that there is no point in not providing another convenient way to download the games for legitimate users.
My point is, it's lame to worry about making some client open source, because users might build a derivative with resharing capabilities, simply because infringers can reshare anyway if they plan to, no matter what the client does.