1. It's not "harsh" to mark anything as spam in your email; the only person it affects is you. The sender will not, of course, even know whether you've classified their stuff as spam.
2. Some commenters are claiming that it's not that much extra effort to visit the sender's site and unsubscribe yourself. This is manifestly not always the case, and more to the point, the user does not KNOW in advance whether it will be the case, when starting the process. However, hitting a plonk button in your mail client will always have a known time-suck-factor in advance.
3. Hitting a plonk button is also, of course, guaranteed to be at least one page-load and one click easier than any possible solution involving visiting a sender's website. Even if there's an unsub link in the spammy email itself, a plonk button is still one page-load less, and easier to find from a UI standpoint. So in sum, plonk is guaranteed to win from on ease of use. Not even a debatable question.
4. Some argue that taking time to unsubscribe yourself via the sender's website is just "the right thing to do", given that you subscribed to their emails in the first place. It seems to me that it should be incumbent on the marketer, not the customer, to make unsubscribing as easy as possible, and to "do the right thing". This should be the default.