I originally brought this up on linux-serial, but I think it makes more sense
that it's part of how printk console device selection works. Without VTs, while
most software is able to handle the situation, some userspace programs expect
/dev/console to still be responsive. Namely systemd. It calls isatty() against
/dev/console, and since /dev/console on VT-less systems currently defaults to
/dev/ttyS0, and when /dev/ttyS0 is disconnected, the ioctl's fail, and it
refuses to write log messages to it.