The whole point of having a quick, online opt-in and an elaborate "mail us a notarized letter" opt-out is to make it very easy to opt in. Why would they want to make it harder? They're already on dodgy legal ground, and the "enter your PIN" wouldn't make it much firmer.

You're thinking like an engineer given the problem of "get people's consent" instead of like a businessman with the goal of "altering the deal."