Okay, so there's two requirements for any legit authorization to pass: identification, and authentication. For Unix accounts, the username is the identification bit, and the password is the authentication bit.

Now, you don't usually type the username when you run sudo, do you? That's because most of the time, the username can be gleaned from context. For example, "Which user is this process running under?".

So, if TouchID's purpose was just to supply identification and not any verification that the identification is legit, that would be pretty pointless.