Often these "spam" reports by end users are just accidental clicks as well. Many of the abuse reports we get are like an email from someone's Mum and visibly legitimate. At other times there are users who use the Report Spam function as a kind of inbox management tool - a way of moving mail away so they don't have to see it because Trash or Delete or whatever is just further away from their pointer.

I tell my friends and family to never click unsubscribe links, unless they had proactively subscribed. Buying something from a company that requires an email does not count. unsolicited marketing emails are spam and should be treated as such. Double so if that company sends marketing emails disguised behind support@company.com.

"Report spam" is quicker and easier than "unsubscribe".

Gmail added a popup asking the user if they want to unsubscribe when flagging a newsletter with the appropriate unsubscribe headers, so it must be common enough to warrant Gmail developer attention.