Handbook states that in order to enable login via serial console one must edit /etc/ttys:
"3. Edit /etc/ttys and change off to on and dialup to vt100 for the ttyu0 entry. Otherwise, a password will not be required to connect via the serial console, resulting in a potential security hole."
This is not necessary because of "onifconsole". Tested on 10.2 but this flag was introduced with 9.3.
Link to this page: https://www.freebsd.org/doc/handbook/serialconsole-setup.html