Hello manrickl,
If you have an employment contract guaranteeing you a certain number of hours per week, they have to pay you for those hours. However, if you are an "employee at will" then you can be cut back on your hours if they don't need you at that particular time.
I know that's not the answer you wanted to hear, but I'm afraid it is legal for them to do this.
Lawyer (JD)
Juris Doctorate Degree