Google has found this to be a very common behavior and thus is introducing a new auto-unsubscribe feature to Gmail to try and streamline the process. Now, when you report a newsletter as spam, you will see the notification box above asking if you want to automatically unsubscribe as well. The feature may not work for all mailing lists, according to Google, and it can take several days for the request to be processed. Still, it seems like a really useful feature to have.
Additionally, the search giant says Gmail won't display the unsubscribe option for lists that are known to be spammers so that you don't get more spam from them by inadvertently verifying that your address is legitimate.