Or, worded another way: the user really doesn't give a damn about your subscription.
If they can't be bothered enough to take a few seconds to open one e-mail message and click on a single link, then whatever you're offering obviously isn't something that they actually want.
As a non-marketer, it seems to me that marketers would want something like double opt-in -- just so that you can ensure that those on your list are people actually interested in $product. Or is it more about the quantity of users on your list and not the quality of those users?