Why is the burden on me to prove people wanted to receive messages?
If you're accused of spamming and you're either using your own mailing list software or manually adding recipients to your list, we'll ask you to provide us with records showing that your list meets the requirements of our spam policy.
Sometimes people don't understand why we require this. People tell us "I know this person really signed up and has just forgotten; surely that's enough of an explanation." Unfortunately it's not: every real spammer claims the same thing, and in some cases we may need to provide your records to other ISPs or blacklist operators to prove that you aren't sending spam.
(Also, even if you're well-intentioned, this response doesn't prove you aren't accidentally spamming people because you added the wrong address to your list by mistake.)
So the reason the burden is on you is a practical one: you're the only person who can prove it. The person complaining can't possibly prove that he didn't sign up for your list, whereas you can prove he did if you keep proper records and verify subscriptions. As a mailing list operator, the responsibility for managing these details rests on your shoulders.
In short: It's definitely in your interest to be able to prove to anyone that you're not a spammer, even if it means a little extra work. It's the only way you can protect yourself.