I doubt this explanation. I have personally seen spamhaus blacklist a newsletter that was subscribed to using double opt-in. What happened: the e-mails never bounced, the provider just closed down and - what a "coincidence" - the domain ended up being owned by someone who immediately used it as a "spam honeypot" for spamhaus.

It's apparently impossible to protect a sender against this by other means than detecting real user interaction. Alternatively, blacklist providers doing this could be ignored...