All work should be paid.
But agencies themselves take the p*$$ out of freelancers all the time.
(I'm a music producer, I know allllllll about agencies taking tha p*$$!!!!) ;-)
The general problem is two fold :
a) There's too much competition on all fronts. When too many vendors are competing - the price sinks to zero.
Basic supply and demand.
b) It's an industry with very few scruples.