It isn't. Wikipedia says this particular type of price fixing is often allowed, and called "Resale price maintenance"

It's the agreement between a manufacturer and a retailer about the price of a product, not an agreement between multiple retailers.

That should still be illegal. I thought manufacturers can only set a "recommended retail price", but retailers can set whatever price they want.

Plus, in this specific case the manufacturer (Apple) is also a retailer.

In Europe even a "recommended retail price" may be an issue in competition law.

MAPs are why you have 'add to cart to see the price' for many items.

They aren't fixing the price to the consumer. They are agreeing not to advertise a price below a certain value.