Because it's a classic EEE-style technique. You force (or very strongly encourage) customers to use your system instead of any competing system. Then, only when all the customers are using your system, and all the competitors are out of business because they don't get customers any more, you rug-pull the customers for money. This wouldn't be a problem if you always had to specify a system when pulling an image, because Docker would be on equal ground to everyone else.