Terminal Passwords in AppleScript
This authenticates the user and unlocks if necessary the old keychain. It's by design, and ensures security practices are followed.  If you really do want to alleviate the users' concerns you can educate them, update the keychains for them, or a whole host of other options.  Bypassing OS X's security in the way you're asking though, I don't believe is possible and also isn't a good idea IMO.