Ask HN: Why do sites make me log in after changing my password?

I forgot my zappos password, so they emailed me a link to change it. Upon successfully submitting the change, they provide me with a link to the login form where I must retype my credentials.

Every site does this so I'm thinking it must be a security measure, but what could that possibly be? When I submit a "change password" form, why can't that form also log me in right then and there?

Surely the reason 99% of people use a "change password" form is because they want to log in immediately.