Spammers use List Washing as a method of removing "trouble makers" (those who have filed formal complaints) from their mailing list. This generally does not include those who have simply sent an unsubscribe request. Spammers use unsubscribe requests as a means to validate their mailing list and to create additional lists of address that are know to open and read spam messages. These lists are considered extremely valuable to spammers who sell the list as validated email addresses.
Responsible list managers may use the process of list washing but the term should not be used in this context. List maintenance, merger/purge are better terms to use when talking about responsible list management where all unsubscribe requests are promptly processed and addresses are not added until they have been positively confirmed. The process can also be used to fine tune their lists for the specific needs or desires of their clients/subscribers.