Such live email addresses are very valuable to spammers. So this feature is susceptible to abuse by spammers providing the unsubscribe functionality, but only prioritizing the attempted unsubscribers higher in their mailing lists.
Google (or any other large e-mail provider, for that matter) could significantly decrease the value of an unsubscribe request to a spammer by automatically sending unsubscribe attempts every time an invalid e-mail address is hit with an e-mail with such a link. If spammers can't assume that an unsubscribe response means the e-mail address is live (and further, if the likelihood of a Gmail unsubscribe being a dead address is very high), then the scam no longer has any value to them, and they'll just stop doing it.
...in which case, it's still a useful thing to do, because it helps legitimate companies prune their e-mail databases of garbage addresses.