GNU Libc uses the Name Service Switch (NSS) to configure the behavior of the C library for the local environment, and to configure how it reads system data, such as passwords and group information. BusyBox has made it Policy that it will never use NSS, and will never use libc calls that make use of NSS. This allows you to run an embedded system without the need for installing an /etc/nsswitch.conf file and without /lib/libnss_* libraries installed.
If you are using a system that is using a remote LDAP server for authentication via GNU libc NSS, and you want to use BusyBox, then you will need to adjust the BusyBox source. Chances are though, that if you have enough space to install of that stuff on your system, then you probably want the full GNU utilities.