It's true that I can retrieve the values I set using android.provider.Settings.System.getString(). It demonstrates that the setting is saved somewhere but is just ignored by the system.
Instead of using android.provider.Settings, the system utilises another setting.
As the setting is set according to the SSID of the access point, system on Android 3.x and 4.x. Can I change a single SSID's setting in the same way as Android 2.3 does?