Guessing here - but they are probably relying on game theory / auction theory. They have a built in "sniping bot" - by allowing you to type your highest price, and it will auto-bid for you until that price.
The fear of being sniped encourages you to bid your maximum value, and not just wait and see if you can sneak in a lower bid. This is what all auction sites want.
The fear of being sniped encourages you to bid your maximum value, and not just wait and see if you can sneak in a lower bid. This is what all auction sites want.