This isn't quite how it works. The /etc/passwd file isn't a command interpreter, it's just a plain text file in UNIX, although a very important one. The login command checks the file (and possibly /etc/shadow, or /etc/security/passwd) to verify that the username and password entered are correct. It also looks at the home directory and the shell. If you enter the correct userid & password, it will drop you into your home directory and execute the shell listed in the passwd file. If you enter an incorrect password, /bin/false is not executed, but the program "login" will itself deny access to the system.