How is Muniz giving back more than money?

Even though Muniz is giving the profits to the pickers, which is greatly needed, he is also giving them a voice. By giving the pickers this voice, they are able to allow the audience know of the harsh conditions of people that live in the favelas and work as pickers.

Is his work on some level exploitative?

It does not seem as his work is exploiting the pickers, even though it could have easily gone in that direction. He actually did lots of good for the people and bringing international attention to a  humane and environmental crisis.