Many literally have a link at the bottom that says "unsubscribe". I've received emails from dozens of different services and companies over the past few years and every single one, without exception, has had this feature. It's prevalent enough that my email client even attempts to find it and presents me with a button in the UI if it does.
Notably CAN-SPAM act requires spam to have effective and visible unsubscribe links. While this doesn't impact off-brand Viagra and Nigerian prince spammers, most legitimate US businesses (such as American Red Cross) should be compliant.
