5 credits/year is a full time undergrad course load - why would a full-time 2-year masters require the student to earn 5.0 credits to graduate? 5.0 credits/year describes a full-time undergraduate course load at my school. 
5.0 credits/2 years describes a full-time graduate course load at my school.
Students may not enroll in more than 3 credits/term (6/year or 1 extra class/term).
400 level courses at my school can count as a 0.5 credit toward either an undergraduate degree or toward a master's degree. 
An undergraduate student may take a full course load of 400 level courses. 


My attempt to answer my question:


Perhaps graduate courses require twice the work that undergraduate courses do.
But if that were the case, then one could take mostly 400 level courses and do nearly half as much work as she would do if she took graduate courses. 
Perhaps 400 level courses require the same amount of work (say twice as much) as lower level graduate courses require. 
But if that were the case, then an undergraduate student who enrolled in a full course load of 400 level courses would have enrolled in a course load equivalent to 5 credits/term when the university calls 3 credits/term an overload and prohibits students from enrolling in more than 3 credits/term.


So why would a full-time master's program require a half-time course load?
It's not clear here if you think a 'half-load' of courses is too high, or too low for a Master's. 

Here's a perspective from my university. Here, your course work serves two purposes in your masters:


Course work is there to rectify gaps in your background. As a school with a huge international student population, we have a huge amount of variance in skill-sets. As a result courses are there partially in order to help make up for missing background, or to get the students onto more common footing. 
Introduction to topics. Here you're admitted without having to declare a supervisor or a topic. The first year of courses serves to introduce students to topics and professors-- possible supervisors. 


After two semesters, most students have paired off with supervisors, and have started their research, which will dominate their time for the subsequent 16+ months, until they defend their thesis.