In Apple's defense, I would imagine that most people who are getting rid of old Apple devices don't think about taking the devices to the Apple Store because that's where they buy new products. They typically take their products to an electronic recycling center, or even worse, they just throw it away in the garbage. Apple may be 'cheating the system' by paying these companies to receive credit for some X amount of pounds, but it's not like they're able to force their customers to return old Apple products to their stores.
I don't see this as cheating, but we're talking about the company who convinced people "Rose Gold" was cool. If they told their customers it would be super for them to bring old iDevices back to the Apple store for recycling, I'm confident they would succeed.