Is it normal for math grad students to be required to take summer classes to work? My mathematics department apparently gets pressured by the university to have more people take courses during the summer.  We are also encouraged to take on TA responsibilities to make extra cash during the summer which is great.... except that they have this rule that if we want to teach/grade (or do any paid work during the summer) we have to enroll  in courses.  The options are slim and so is the stipend; I'm a grad student currently trying to prepare for my final prelim exam and do some research during the summer; I'd like to make a little extra cash.... but I crunched the numbers and it looks like I'll net about $600 for an entire summer, not exactly a living wage.  

I should also add that our department (mathematics) brings more funds (in course work and its associated tuition) than any other department (I'm sure biology/chemistry pulls more in research grants), .... so it seems very tawdry of them to "get money back" by forcing us to enroll in classes.

So my question is: Is this normal?  I've spoken with some of my professors and they said it is not; but just curious what this community has to say and if this is ethical or are there any statistics on this sort of thing.
Having observed several places in several different decades: summer funding is always weird and maybe non-existent or not-a-good-deal.

During the 9-month academic year, in the U.S., yes, indeed, people working as TAs/RAs have to be registered as students (because as other people said, these are student jobs), and this entails fees. Most often, tuition is included in the "benefits", but not invariably. 

At every university I've been, grad students registered in the 9-month academic year did not have to explicitly register, nor pay tuition, in the summer, in order to work as summer TAs/RAs. This part is quite anomalous, and looks like a way for some layer of the institution to suck money out of people.

My experience indicates that there's never enough summer support to go around, which effectively creates a labor surplus, which, predictably, leads to certain abuses. For that matter, the usual double-time rate of summer classes is pretty nasty, and not fun to see. A bit taxing to teach, but almost uniformly near-disastrous for the students, who cannot simply "decide" to assimilate things twice as fast.

In some cases, the funding and flow of money for summer teaching/classes is different from the 9-month year, so it is not necessarily deliberately abusive, though possibly irresponsible or negligent, of your dept to not tell you your net pay. If it's really just 600 dollars, it's a ridiculous situation, yes.

But the net pay should be substantial, not next-to-nothing. That aspect strikes me as odd.

In summary, the situation you describe is unusual in that the net pay is so low. Having to sign up for courses and pay summer fees (or, worse, tuition) is unusual. The vagueness is less atypical, since sometimes control of summer classes is not in the hands of the department.