Under US law, it is not mandated that transactional email notices must contain an unsubscribe link. But is it a bad idea to include one anyway? If you don't include an unsubscribe link, you run the risk of sending that mail to the wrong person and leaving them with no way to make that unwanted mail stop. And that quite validly considered spam by the recipient! That person receiving that message didn't opt-in to it, didn't sign up for it, and isn't a registered customer. You shouldn't have sent it to them in the first place, but the very least you can do is give them a way to make it stop.
I've seen transactional notices both go to the wrong people and to spamtrap addresses. And let me tell you, I know from experience that a savvy spam filterer like Cloudmark is not necessarily going to give you a free pass on spamtrap hits just because your messaging is transactional. If you want to remain on the good side of entities like that, you need to make sure you're doing things like validating addresses, respecting bounces and suppressing non-responding addresses. And let's not forget, make sure your support knows how to handle a "this is the wrong person" email issue.