I hold a BS in engineering and teach high school math for fun (and love of the subject) in a right-to-work state. My previous salary was more than double my teaching salary.
My state requires high school teachers to major in the subject they'll teach and minor in education. This has resulted in a terrible shortage of math and science teachers - why work so hard for the degree and then take a low-paying teaching job? (Not everyone has a well-paid husband like mine!)
By lobbying against teachers being paid more for teaching some subjects, unions do (some) of their members - and all students - a terrible disservice.