3 courses per semester is a lot of work if you're doing it right, especially when you consider the amount of time research can take. 8-10 hours per week is a good estimate of the amount of time each class takes.
Also, very few professors make $150k-- those are the stars-- and they tend to be in their late 30s if not 40s. Academic pay is 1/5 to 1/10 of what it is in other industries (and yes, it's harder to become a tenured professor than to make $1 million/year on Wall Street).
American tuition is ridiculous, but overcompensation of teaching staff is not the problem. With tuition at $40,000/year for 10 classes, that's $4000 per class. If average class size is 25, that's $100k for 150 hours of the professor's work ($667/hour). So a professor is bringing in $500+k through teaching alone. For that professor to be making $65k (more realistic than the $150k figure) is not overcompensation.