It's not the same as your myUnisa password. Osprey is not designed/created/administered by the myUnisa team. That's why it actually does stuff that students actually want.
If you read your tutorial letter 101, you'll find the password for Osprey.
Hint: It's generally a good idea to read every tutorial letter you receive, there's usually some important stuff in there. A lot of the frequently asked questions can be answered if people just read their tut letters, eg. "What's the password to Osprey?".
To answer your second question, it's for security reasons. Only registered students should have access to current material. Your myUnisa and Osprey forum passwords don't change from year to year and you don't need to be registered to keep using these services but the document repository is for current students only.