That's because non-interactive changing of passwords is a dangerous thing. Don't go there unless you're absolutely sure what you're going to do. If you mess up, *NOONE* will be able to log anymore, and you'll need to do some massive recovery. Do you have your backup tapes ready? Do you know which steps to take to recover your system? Have to checked that your backup procedure is actually working? Can you read from your backup tape?

Here's a hint (I ain't going to spoon feed you on things changing the passwords, if you need handhelding, you shouldn't be doing this). /usr/bin/passwd reads directly from the (pseudo)-terminal, not from STDIN.

Abigail