I only noticed it because I got bored and looked through all the settings. There's also an obscure option to tell Android that a particular wifi point is a mobi

I only noticed it because I got bored and looked through all the settings. There's also an obscure option to tell Android that a particular wifi point is a mobile hotspot (i.e. should be subject to the same data restrictions as the 3G/4G connection). I have no idea how many apps pay attention to the setting though. Bizarrely it's under data usage > more options > mobile hotspots, rather than being an advanced option on wifi networks.