splash.initrd doesn't support booting by label. Booting by label gets around not knowing which device is the root device. As an instructor I boot BT on a variety of PCs and I don't know what the root device is going to be until I boot. On some PCs the root partition of my usb hard drive is sda1, on others it is sdc1. Once during the semester the BIOS on all of the PCs was updated and the root partition changed from sdb1 to sda1. If root=LABEL was working then I wouldn't have to worry about this.

When I did a apt-get upgrade one of the packages was initramfs-tools. When it was updated a new initrd was generated and it supports root=LABEL so it is possible on ubuntu. Slax couldn't do this as their version of busybox is too old.