/sbin/ and /usr/sbin/ are in the $PATH environment set if the user becomes root by invoking a root login shell using "su -". As only root is allowed to start, stop or restart services it is absolutely sufficient that both directories are only set within the $PATH for root user and not normal system users.
Except that there are a number of programs in that directory which can be used to return useful information to unprivileged users, such as ifconfig and route. If you like to track system state without assuming the risk of being superuser, add those directories to your path. I do this for all users by commenting out the "if" and "fi" in /etc/profile bracketing the calls to pathmunge. Issue those calls unconditionally.
