Right now this is baked into the aufs-root mount intentionally, as we are setting up with a limited amount of space for users to make changes in and don't want to eat it up with logs over time. If you are troubleshooting and need logs to be persistent, you can always boot in generic mode (see
http://ubermix.org/customization.html for instructions), which leaves the mounts in a relatively stock state (ie /var/log is written to the disk). You could also set the machine to boot in generic mode permanently by editing /boot/grub/grub.cfg. Of course, this defeats much of the reset functionality.
I initially tried to make the tmpfs mounts part of a more standard fstab, but found that mountall was too unreliable with an aufs-root and would hang on tmpfs mounts.