There is unrar-free, but this is too immature and has non standard CLI API.
Here’s what you have to do – build it for yourself! :-)
wget http://www.rarlab.com/rar/unrarsrc-5.2.5.tar.gz tar xfvz unrarsrc-5.2.5.tar.gz cd unrar/ make sudo cp unrar /usr/bin/unrar-nonfree sudo rm /etc/alternatives/unrar #might fail, don't worry sudo ln -s /usr/bin/unrar-nonfree /etc/alternatives/unrar
From now on, you have system wide unrar cmd available to use. And SABnzbdPlus won’t complain :-)