Back in the old days I was a grad. student teaching assistant, around the time the Univ. of CA 'student academic employees' were unionized and became part of UAW of all things. Being a liberal at the time (in a decidedly unliberal place -- computer science department at UC Irvine) I signed up as a member of the union.
Of course being a TA you didn't have to join the union, it was voluntary. but if you didn't you still had "agency fees" (somewhat less than union dues) deducted from your paycheck and paid to the union. What the union then did with everyone's "agency fee" dues is anyone's guess.
Nowadays, in a similar situation, I would not join the union. Therefore I will be voing for 75 because i bet a lot of people who aren't union members are paying "agency fees" that they can't opt out of without giving up their jobs.