Okay I'll accept that Apple may not be doing the recycling themselves but just like going to your local city dump you have to pay a fee to dump your trash. It's costing someone money to do the recycling. There is definitely some cost involved. Also keep in mind even if there is no money involved on Apple's part can you imagine how much trouble it is for any company to take back chargers from customers? I have a drawer full of them.
The member
@russell_314 was mentioning earlier to limit the amount of adapter trade-ins but customers scam companies and that wouldn't work. For example, customer JOHN was told by Apple that he can trade-in a maximum of two adapters per household. Well customer JOHN will have his wife and their kids do trade-ins under different Apple I.D's in order to get rid of the pile of adapters they have in their household and they will get credit for each. This ruins it for honest customers.
That's why it's not "financially" worth it to Apple or any company to have a trade-in program for adapters.