The solution should not be to make science classes easier or less work, but to increase the difficulty of humanities courses. The culture of hard work is important to establish, whether you are a humanities student OR a sciences student.
As a science student, I routinely loaded my schedule with humanities courses and hardly had to spend time on them (for the most part). There is no good reason why these courses were so easy in comparison to science courses, in my opinion.
And yes, I agree that decreasing the average GPA in humanities classes is better, but you're ignoring one thing; students are horrible complainers. If you give them a poor grade, they will whine and wheedle. If it's on a test, and their answers are demonstratively false, they have no room to argue. If it's a paper, they will plead and annoy you until the cows come home.
Simply put, poor grades in a science class are much more defensible because science isn't as subjective. It's hard to grade hard in a humanities course.