Of course they're allowed. When you run a business, you set the price of the items you sell.
Not if you are buying direct from say ASUS. They have MAP pricing.
MAP is the MINIMUM that you can advertise the price for to protect people from backdooring and driving prices down. They don't give a flying fuck if you sell it for $5,000, but if you ADVERTISE IT below what they tell you is allowed, they pull your seller status.
And that's why you see sites saying "Add to cart to see price" - because it's a way around MAP - MAP was $400, they want to sell at $350, they can't advertise it @ $350, so they say add it to your cart so you can see the price.