There's no such thing as a free lunch, regardless of how you distribute the costs. If a seller is refunding the cost of shipping on returns, they are passing that cost on to you in some other way (higher prices, membership fees, etc).
You could make the argument that if you return items more often than the average customer, your share of those costs may be getting subsidized by the customers who return items less often. But if you return items less often, you're probably worse off.
You could make the argument that if you return items more often than the average customer, your share of those costs may be getting subsidized by the customers who return items less often. But if you return items less often, you're probably worse off.