The reason is simple: it works. Many people will hand over their email address with no guarantee of anything in return. A shockingly large number of people will do that. And then the opt-in spam begins. It's a never ending email campaign for who knows what. Does the spam work? Surprisingly, yes. Enough to be worth paying to keep sending it out. Legalised spam.
Is an email address worth something? Gather enough of them and yes, absolutely it is.
This is all pure speculation. Or is it?
If I had a dime for every site that collects email addresses without delivering anything first...
As if Mozilla didn't have enough data about users already.
Anyway, if you wanna see what this thing is here you go:
http://github.com/mozilla/napkin/zipball/master
Unzip the file and start reading.