The "Apache SpamAssassin: Forced Global ON" option in WHM's Exim Configuration manager does not actually enable SpamAssassin the way you think it does. It creates the SpamAssassin module/option in the cPanel accounts, however if no configurations are set by the end user it's not really doing any good.
The article will cover how to enable SpamAssassin for every cPanel account account on your server with a few steps.
This article requires common knowledge of SSH / Terminal usage.
touch /etc/global_spamassassin_enable
touch /root/support/spamassassin/user_prefs && echo "required_score 3" >> /root/support/spamassassin/user_prefs
for i in `cat /etc/trueuserdomains | cut -d: -d' ' -f2-` ;do mkdir /home/$i/.spamassassin && touch /home/$i/.spamassassinenable ;done
for i in `cat /etc/trueuserdomains | cut -d: -d' ' -f2-` ;do cp /root/support/spamassassin/user_prefs /home/$i/.spamassassin/ ;done
for i in `cat /etc/trueuserdomains | cut -d: -d' ' -f2-` ;do chown $i:$i /home/$i/.spamassassinenable && chown -R $i:$i /home/$i/.spamassassin ;done