The reason is for how returns are handled. If Apple gave you the card, you could return the Mac and get the full value of your purchase price back, and keep the gift card. By discounting the Mac $150 and charging for the gift card, you need to return both to get all your money back. Apple doesn’t say it’s a “Free Gift Card,” just that you get one. If you don’t do a return, the $150 Mac discount covers the cost of the gift card; so you are getting an extra $150 gift card, as per the promotion, as explained here:
Qualified Purchasers shall receive a discount equal to the value of the Apple Gift Card off the price of the eligible Mac or iPad, but will be charged for all items in their cart, including the Apple Gift Card.
