Unfortunately, we have nothing to do with the writing of any programs. 'Forget Site' should only remove the things like settings and other such data. There would be no reason to remove password data as well.

A bug has been filed: 702925 "Forget About This Site" shouldn't delete saved passwords (without a prompt?)

In the event that the passwords information is lost, you should record all information in a separate text file somewhere else on your hard drive, or written down. You can easily copy and paste if you need to do so. If you are concerned about someone else looking at that file, you can compress it using a password.