The reason why Apple has quota is:
1) to ensure that seller of their products do not pull a bait and switch.
That is, for example, if Hauwei pays higher incentives, a seller can say, carry the iPhone, then convince the customer to buy another product.
Basically, they use Apple's popularity and ad budget to attract customer, without being serious about selling their product. By implementing a quota, it forces the re-seller to put their products front and center, not hide the product at the back of store. (I believe Coke and Pepsi does this alot)
2) To regulate prices. Apple wants to keep the prices mostly constant (they don't want their products to go 'on sale').
So they sell the product only at a slight discount to the re-seller. (So for example, a retail $699 iPhone may be sold at a discount of $650, hardly any profit). This maximizes Apple profits and the re-seller gets hardly any profits until they reach a specific quota; or in the case of carriers, you get to carry the phone (to attract customers) only if you buy a large amount.
The carrier cannot have inventory sitting around, so they must sell at near cost or slight loss forcing them to promote the iPhone. Similarly with the retailer, will sell at near cost/loss to reach their quota.
The result is most iPhones end up sold at almost the same price $650-$699 (in this, example).
No idea if it's any of this legal or illegal, but it's used by alot of companies.
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