Why is it safe for 1password to autofill my master password when signing into 1password online?

It can only autofill if you unlocked the browser plugin previously, so you entered your password before, so it's safe. If you don't want people using your laptop autofilling your credentials, you need to lock 1Password when you switch users, which you can do manually or define automatic locking of the browser plugin. In case you also have the desktop app installed, you find the lock settings in the desktop app instead.