According to the Filesystem Hierarchy Standard the /bin directory should contain utilities needed in single user mode. In practice, many Linux distributions make the directory a symbolic link to /usr/bin. Similarly, /sbin is nowadays often a symbolic link to /usr/bin as well.
What's the rationale behind the symlinks?
Short summary of the page suggested by don_crissti:
Scattering utilities over different directories is no longer necessary and storing them all in /usr/bin simplifies the file system hierarchy. Also, the change makes Unix and Linux scripts / programmes more compatible.
Historically the utilities in the /bin and /sbin directories were used to mount the usr partition. This job is nowadays done by initramfs, and splitting the directories therefore no longer serves any purpose. The simplified file system hierarchy means, for instance, that distributions no longer need to fix paths to binaries (as they're now all in /usr/bin).
