That's how price of retail SHOULD be set, but here's the thing. Retailers are smart enough to know that they're not going to make much money if they sell their product in stores for $40, while the exact same product is sold online for $30. They'll simply refuse to carry the product at all, because the retailers are well aware that the sales from retail still remain a significant chunk of profits that the developers/publishers would be foolish to pass up. Other companies would be more than happy to take the shelf space.
That's why retailers can still insist on price parity between digital and retail copies. If retailers refuse to carry the title, then regional distributors have nowhere to distribute to. No regional distributors means no translations - even for digital versions, which means even less sales.
