There's the point-- the activity fees, etc.
What about forcing students to hand money to organizations they object to on religious or moral grounds?
Has this already gone through the courts?
I'm not quite sure what you mean. Can you give an example of a group or organization that a student might be compelled to join, but they might have philosophical or religious differences with?
Besides tuition, parking, housing books and other matriculation fees, I can't think of any other mandatory "group" fees.