I don't know if that's quite true. I think in a script in the bottom is fine, as thats where people expect it to be. If its a simple, one-click thing, then its fine. I think adding big unsubscribe buttons at the top is just noise and obstructive to people that are actually reading your emails. Sure there are people that will just mark your email as spam or filter it to the trash, but I don't know how much of a difference the location of the unsubscribe button will make to them because I suspect they likely aren't even opening your emails to begin with.