The Debian package of stunnel changed the default location of the binaries, from /usr/sbin to /usr/bin. This is because we consider that stunnel is not an admin only tool, but is useful to regular users (as it does not require root privileges for several common use cases).
I did the same in Cygwin (or maybe the previous packager did; anyway Cygwin puts the main executable in /usr/bin), for the same reason.