Scalping also actively damages the pricing, which is part of the product. Valve wants to sell this product at a specific price, which is targeted to an audience. By scalping and ultimately changing the price, you are hurting both the consumer, who now pays more, and the company, who doesn't see a cent of this increase and is now failing its target.
Scalping also damages the demand for the product, since it creates a submarket that is volatile and unpredictable.
Scalping is a bad thing because by basically any measure, a market with scalping is worse for everybody involved than one with scalping. Except for scalpers, who make money off it by making it worse for everybody else. Which is why scalpers are bad people.