Good question!
Perhaps according to the Yesod of the Nesivos Shalom in e-mail #341 on
this page, maybe "ain maspikin biyado la'asos Teshuvah" because he doesn't NEED teshuvah since while sinning his heart is broken and he is thinking of Hashem...
That's a little Dochek though.
Here's another Pshat... I would say is that in the case of addiction this chazal doesn't apply in the literal sense, but in a figurative sense... What do I mean? Once we have this illness, we will tell ourselves "echte ve'ashuv" for years. But that doesn't mean Hashem won't help us HEAL from our sickness. And once we are healed, we will see that Hashem took care of the teshuvah for us. "Ain maspikin Bi'yado" - but "Yesh Maspik Bidai Hashem Le'hashivo"!!
I ask you, where would these Torah words come from if not for GYE? ;D