They did it because it was the easiest way for google to make it easy to filter out encrypted search without breaking their other services. A very simple change for google -vs- a very difficult firewall configuration for a network administrator. (You can't just block https://www.google.com/ without breaking a whole bunch of other services you may want to be using) (and you probably don't have the budget & manpower to deploy the type of network you would need to scan & filter HTTPS traffic as an organization if you are a school, either)