“Your email can be marked a spam for a variety of reasons, whether someone was frustrated that they couldn’t find the unsubscribe or maybe they flat out never signed up for mail in the first place. Some recipients may be more justified in using the spam button than others, but it’s an email marketers job to watch their spam report complaints and make adjustments to their program accordingly — your subscribers and your deliverability, depend on it!”
Use feedback loops, such as those provided by Gmail, to determine who is marking your emails as spam. Do this regularly to remove disinterested folk from your list and maintain company credibility. You don’t want to end up in spam reports on a consistent basis.