It looks like busybox is compiled with bz2 enabled but I didn't found that it is used anywhere.
bzip2 is widely used for building packages but OpenWrt doesn't use it.
I created a PR https://github.com/openwrt/openwrt/pull/3753 that disables /usr/bin/bunzip2 and /usr/bin/bzcat
In BB config said that this may save up to 8.7 kb