Hello.
Probably because 'Live System User' has no password, lightdm is broken
for keyboard navigation in the live-cd.
If it had a password, when choosing the user the focus would jump to the
password field and then one could hit enter or navigate to the other
buttons. The way it is now hitting 'enter' just expands the user options
and there's no way to change focus to the 'login' button.
Anyway, this seems a limitation of the gtk greeter. Is it planned to
skip the DM screen altogether like in previous versions that used gdm?