Important, but unanswered questions (because they are hard, so let's ignore them!):
(1) How many of these users just mark all of the newsletters as spam?
(2) How many users don't realize that they can dismiss the dialog, and enter their email because they think that they are forced to? How many of these are really "engaged" users vs. just users that are willing to jump through a couple of hoops before they disappear in the ether?
(3) If those extra sign-ups are due to people thinking that they have to enter their email, how is this any different (ethically or practically) than actually forcing them to enter their email? Are you only making this distinction in the smallest way possible so that you don't run afowl of anti-spam laws?
(4) Did the A/B test include a "No thanks, I want to read the article instead" instead of just a "No thanks" button? How much does that affect the number of sign-ups?
(5) How many of those users will actually open their newsletters vs. getting lost in the clutter of other newsletters that they automatically sign-up for?