I sign up for hundreds upon hundreds of lists. I maintain multiple "hamtraps," collections of received mail that I actually asked for. So it's not spam, but sometimes the line gets a little blurred.
Take, for example, a random veterans affairs site. In April I signed up on their site, but never completed registration.
In the past thirty days, they've sent me five requests to complete my registration. They may have sent me more requests to complete; I don't know, because Gmail claims to empty out my spam folder every thirty days.
Yup, they're going to the spam folder at Gmail.
I have some idea why. It's for something they did. Or rather, something they won't do: They won't let go.
If you keep sending mail to unconfirmed signups every week, you're driving people nuts. People who don't want your mail, so they're reporting it as spam every single time. People who didn't complete because they don't want to complete. Maybe sending them a second nudge to complete was OK, but five is far beyond what I'd call an acceptable best practice.
Is it legal? Absolutely. Is it blockable? Absolutely. It wouldn't suprise me to find that they were having delivery issues at other ISPs, not just Gmail. ISPs, especially the big dogs (AOL, Yahoo, Hotmail) do not take kindly to senders who generate complaints, and it seems very likely that this practice does exactly that.
If you want to be a good sender, confirming your list is great. Asking people to complete their registration is fine. But stop and think: What is reasonable? Five requests (so far, I might add) is overkill. The whole point of confirming is to validate them as a user, counting them as engaged, knowing they want your mail. It's silly, and damaging, to keep nudging people over and over and over, if they're clearly choosing not to join this group.
As a sender, you greatly improve your deliverability by jettisoning non-responders. If you keep pinging them repeatedly, you're denying yourself the benefit of this process, and ensuring that ISPs are going to block your mail.