Is it illegal for iTunes to access all my passwords(FB,TW,etc)

I have made a backup to my Iphone5(encrypted), i did not put a password to it, but when i try to Restore from that backup, it asked me a password, i searched on web and i find that this password was the one from my Facebook account(it is not stored in Keychain or enywhere else), how did my FB password could be used on my iPhone BACKUP? Is this a bug or something illegal?