Google Search Appliances must enforce password minimum lifetime restrictions.
Password minimum lifetime is defined as: the minimum period of time, (typically in days) a user's password must be in effect before the user can change it.
Restricting this setting limits the user's ability to change their password. Passwords need to be changed at specific policy based intervals, however if the application allows the user to immediately and continually change their password then the password could be repeatedly changed in a short period of time so as to defeat the organizations policy regarding password reuse.
This would allow users to keep using the same password over and over again by immediately changing their password X number of times. This would effectively negate password policy.