Ahh, but then if it's a secure download link and the security is not built into the client, then all it takes is one malicious customer to offer it via P2P and everyone will be able to play it anyway.
Bittorrent is a fantastic way to distribute software (look at WoW patches for example) at release, (its effectiveness dies off later), but only if the authentication is built into the product at the client end. You should'nt have any reservations about not running Bittorrent - if you're not actively seeding/downloading, it does nothing.