[CentOS] Login Console on ttyUSB0

Kernel parameters aside (console=ttyUSB0 console=tty0), I am trying to get
a login console working on a USB-to-serial adapter.

systemctl enable serial-getty at ttyUSB0.service
systemctl start serial-getty at ttyUSB0.service

This works without issue on Debian 8. However when running the same on
CentOS 7, my logs are filled with:



May 16 12:44:58 localhost.localdomain systemd[1]: Starting Serial Getty on
ttyUSB0...
May 16 12:44:58 localhost.localdomain agetty[1842]: /dev/ttyUSB0: cannot
open as standard input: Permission denied
May 16 12:45:08 localhost.localdomain systemd[1]:
serial-getty at ttyUSB0.service holdoff time over, scheduling restart.
May 16 12:45:08 localhost.localdomain systemd[1]: Started Serial Getty on
ttyUSB0.
May 16 12:45:08 localhost.localdomain systemd[1]: Starting Serial Getty on
ttyUSB0...
May 16 12:45:08 localhost.localdomain agetty[1845]: /dev/ttyUSB0: cannot
open as standard input: Permission denied
May 16 12:45:18 localhost.localdomain systemd[1]:
serial-getty at ttyUSB0.service holdoff time over, scheduling restart.
May 16 12:45:18 localhost.localdomain systemd[1]: Started Serial Getty on
ttyUSB0.
May 16 12:45:18 localhost.localdomain systemd[1]: Starting Serial Getty on
ttyUSB0...
May 16 12:45:18 localhost.localdomain agetty[1850]: /dev/ttyUSB0: cannot
open as standard input: Permission denied
May 16 12:45:29 localhost.localdomain systemd[1]:
serial-getty at ttyUSB0.service holdoff time over, scheduling restart.
May 16 12:45:29 localhost.localdomain systemd[1]: Started Serial Getty on
ttyUSB0.
May 16 12:45:29 localhost.localdomain systemd[1]: Starting Serial Getty on
ttyUSB0...
May 16 12:45:29 localhost.localdomain agetty[1853]: /dev/ttyUSB0: cannot
open as standard input: Permission denied
May 16 12:45:39 localhost.localdomain systemd[1]:
serial-getty at ttyUSB0.service holdoff time over, scheduling restart.
May 16 12:45:39 localhost.localdomain systemd[1]: Started Serial Getty on
ttyUSB0.


I thought this might be a permissions issue. But comparing the permission
on the debian box to the centos box yeilds the same results. When no one is
logged in on the serial line on both machines, the permissions on the
device node are as follows:

crw--w----. 1 root tty     188,  0 May 16 13:49 /dev/ttyUSB0
(root:tty 0640)

Any ideas as to what is blocking the serial login on the CentOS 7 box?