Did Walmart provide you with the ability to
The part you're missing is what we call in the retail world a "loss leader." Apple distributes millions of apps per day for free to the consumer and developer. Apple pays all costs associated with this distribution. Why? Because it brings in customers. It's a HUGE reason why the App Store, and others like it on other platforms have become a success. How can Apple do this? Because it takes a cut of the profits from other Apps to make up the costs. Just like how Spotify takes the hit on free subscriptions. It brings customers in, and you make up that loss in other ways.
Let's go back to the Walmart analogy everyone keeps making. Walmart runs an ad for $25 off a new video game. Awesome! You just saved money! They lost money on that video game. But Walmart is banking on the fact that you're going to buy your controllers (Spotify) at Walmart, where they will make up the loss.
Sure you may be able to go somewhere else and pay less for your controllers, but there's no guarantee that you would, but Walmart made it so convenient for you, so they should rightfully get a cut of the controllers.
Walmart may also have their own brand of controllers that they sell at a discount or even the same amount of money. HEY THATS UNFAIR! Is Walmart paying an extra cut for their controller? No, because they made it and distributed it. You're more than welcome to sell your controllers only at Winn-Dixie (android), but then YOU are losing customers.
But why should Walmart get a cut of every controller a customer buys from them? They obviously like the controller, because they keep buying them. Walmart distributed it, and is continuing to provide you the avenue to sell them. Pay your share, or go somewhere else. If you started putting notes in the packaging saying "Don't buy another controller from Walmart! Buy it from us online and save money!" Walmart would have every right to stop distributing your product. You have broken a good faith relationship with the company that originally got you that business.