Auto-opt-in?

Bronto’s Chris Kolbenschlag frames the discussion well: He purchased from an online retailer, they assumed he wanted to receive followup emails, and thus, those emails did eventually commence.
This is something I’ve had a lot of experience with. Working for an e-commerce service provider from later 2000 through mid 2006, I was the guy setting permission policy, dealing with spam complaints and advising on deliverability issues, primarily regarding email lists built over time from online store purchasers. There was an opt-in checkbox on the platform’s checkout pages, and it was up to the client as to whether or not it was pre-checked (“opted-in”) by default. Most clients pre-checked it by default.
My experience was, from a deliverability perspective, this kind of auto opt-in didn’t really present issues. People didn’t tend to forge addresses when purchasing, and people tended not to report mail as spam when it’s coming from somebody they just did business with.
I’m not saying it’s the wisest way to do things, by any means. If you have any other deliverability challenges at all, this kind of thing could likely add to them. And is it the most consumer friendly way to run things? I don’t think so. In my humble opinion, it’s always better to wait for the consumer to sign up on their own. But I’m not one of those aggressive marketer types.
And of course, the laws governing email permission vary by locale.