They did it this way because they believe they know what's better for the customer. WiFi is used for more than just surfing the web and turning it off negates those functions. It's also just as easy to forget to turn it back on when you pull your phone out to look something up, potentially costing people money. Leaving it on in the background shouldn't be burning your battery and if it's trying to grab networks on your commute, there's options in the WiFi settings menu to disable that functionality.