They're paying for it with a slightly more expensive laptop/computer price. The manufacturers don't get Windows for free, they ultimately pass that cost on to the consumer.
Yes, of course, but I think it's a slightly shallow perspective.
It's a push-pull between the hardware vendor and Microsoft. You can bet that a hardware vendor with large numbers can get a better deal on the Windows license. Why? Because the vendor effectively "sells" you, the customer to Microsoft.