Ubuntu 17.10 & MATE Desktop 1.18.0
Ran into same issue of keyboard backlight always turning on after login.
My temporary workaround is simply to run the following as your normal user (no sudo):
gsettings set org.mate.power-manager kbd-brightness-on-ac 0
also, so that it doesn’t keep turning back on after coming back from idle when on battery:
gsettings set org.mate.power-manager kbd-backlight-battery-reduce false