Is it immoral to expect uncompensated contribution to the university from doctorate students?
In universities in the USA, the type of work that you describe is typically paid for, with the student being employed as a Teaching Assistant (TA).  In some cases, this is an hourly job; in other cases it comes with a stipend and tuition support. The time a student spends per week varies, but in my experience is most typically averages to 10-20 hours/week (typically lower for hourly wage, higher for stipend).

Many departments do require students to spend time as a teaching assistant, under the theory that the work of instruction is an important part of one's education.  It sounds like the universities that you describe are using this theory, but then not compensating the students.

To my mind, whether this is moral or not depends on the nature of the work and the amount of time that is required.  


At one end of the spectrum, if a student is asked to give a couple of guest lectures over the course of a semester, then the experience of preparation and explanation seems like a reasonable inclusion in their educational experience, especially if coached by the professor.
At the other end of the spectrum, if a student is asked to do 20 hours of grading and other "scut work" that has little educational value, then it is clearly a job for which they should be paid.


Somewhere in between there is some grey area, but in general my feeling is that the more routine and less educational aspects of teaching are work for which whoever conducts it should be definitely be compensated.