The artist is not consuming them for free. He spent their time and effort creating something. They are not monetizing what is provided for free, but their creation. If someone wants to pay him for his work, what is it to you?
Separately, based on my life experience, this notion of "exploitation" is truly harmful to those who subscribe to it. You'll never outdo your limiting beliefs. You don't have to take my word for it, but I encourage you to explore who was it that instilled that belief in you and what you stand to gain from subscribing to it. I mean this in earnest, as advice from a stranger, for what it's worth. <3
They could spend the same time and effort arranging blank slides creating the exact same thing without the pictures, and it would have far less value
The value does not come only from the arrangement of the pieces, but also from the pieces themselves. To say the only value comes from the artist is just as wrong as saying its only value comes from the source material.
A painter cannot paint a picture without a canvas, and yet the canvas itself is generally not free, with the canvas-maker earning something from the sale of the canvas itself.
The rest of your comment is wildly condescending, and while I am open to reconsidering my beliefs, I generally do not follow the advice of condescending people. My life experience tells me such people are generally both more close-minded and more morally bankrupt than those who can evangelize their beliefs without condescension. Perhaps consider changing your approach to providing unsolicited advice?
I am very all ears about these galleries, where should I reach out to?