iOS 7 uses iCloud to store your passwords for websites you log into. But sometimes, by default, Safari won't prompt you to save passwords for certain sites—sites that explicitly request that web browsers not save such data.
But they're your passwords, and Apple clearly thinks you deserve a vote on whether your iOS device saves them. Head over to the Settings app, tap on Safari, and then tap on Passwords & Autofill. Enable the Always Allow setting, and Safari will now be willing to save every single password you enter, even on sites that attempt to disallow that option.
Mac OS X Hints