Suffice it to say that they're substantially more aggressive with email capture and viral mechanics than I would be. All of their normal emails have unsubscribe links. It appears their automated welcome email doesn't -- that is probably a technical oversight, since it also misses all their normal templating, too.
They're almost certainly using a different service for transactional emails than they are for bulk emails.
I've used SendGrid and MailChimp together before, and while I'd avidly recommend either of them to people needing that sort of thing, my biggest pain point was always in the reconciliation between the two.
