It may very well be the case that users more often click a link to unsubscribe directly on the site more than they click the 'unsubscribe' feature in the email client (I don't have any stats on that).
The original question remains though: How does the stated service here work when there is only a List-Unsubscribe: header and no web form?
The original question remains though: How does the stated service here work when there is only a List-Unsubscribe: header and no web form?