On Leanpub, readers have to opt-in to share their email address with you.
However, when you publish a new version of your book you can include a message to readers that they will receive by email; this is opt-out.
Leanpub authors and readers are both our customers, and anything that serves one of them at the expense of the other actually serves neither.
Specifically, there are two issues here:
The Principle of Least Surprise
Places like Apple and Amazon do not give you email addresses of readers. This is not even something that a reader would think about. Leanpub is a real store (albeit a smaller one!), not a piece of shopping cart software. So, the same expectation applies, and there's no way that we would just violate that expectation without asking readers. So, it's a question of "opt-in" vs. "opt-out" on the checkbox; simply providing all emails is not even on the table.
False Negatives vs. False Positives
Say email sharing was an opt-opt checkbox. Then say someone doesn't read the checkbox and does not uncheck it. Then you get their email address from us, and you email them.
They will think that either you spammed them or that we spammed them.
This is why opt-in is better: you can actually trust it somewhat. You will get false negatives (i.e. you will not get email addresses from some people who would want you to have them), but that is better for you and for us than false positives (i.e. you getting email addresses that you are not supposed to get).
If our checkbox was opt-out, you could *not* trust that people who left it checked actually wanted your email. So, for any given email you sent, there would be a high probability that a significant % of your readers would be upset with you (or us) for spamming them.
So, it is actually arguably better for you for the email address to be opt-in not opt-out. At least at the beginning, you will be able to trust that people want your email.
Finally, we do have a MailChimp integration as well, to help you email readers that have opted in to sharing their email address with you.