A prominent example is child labor laws. Before these were passed, the market would bear endangerment and injuries to children because something was better than nothing to families that had very little, and it was rationally in each family’s best interest to take the risk in exchange for some small amount of money. If they had all collaborated in an organized strike, they could have demanded better conditions, but in practice that wasn’t going to happen for people that were already extremely constrained on money and time (and all it takes is a minority of defectors to prevent this from working). It creates a feedback cycle of compounding inequality, and the only way to break it is via regulation.
So once you allow unpaid work in exchange for "exposure", that becomes the new normal, and no individual photographer can alter the system while still acting in their own best financial interest.