The regs don't require this - all they require is that they can be turned on and off independantly on the dipped and main beam headlights:
ECE Reg 48
6.3.7 Electrical connections
It shall be possible to switch the front fog lamps ON and OFF independently of
the main-beam headlamps, the dipped-beam headlamps or any combination of
main- and dipped-beam headlamps, unless the front fog lamps are used as part of another lighting function in an AFS; however, the switching ON of the front fog lamps function shall have the priority over the function for which the front fog lamps are used as a part.
If it's a combined unit (e.g. side lights and fog lights) then turning on the fogs will also turn on the side lights. Turning on the sidelights will not turn on the fogs. And vice versa of course.
I see plenty driving round the leafy lanes of Sussex with just fogs on.
Interestingly for rear fogs the regs allow two scenarios:
6.11.7.3.1. The rear fog lamp(s) may continue to operate until the position lamps are switched off, and the rear fog lamp(s) shall then remain off until deliberately switched on again;
6.11.7.3.2. A warning, at least audible, additional to the mandatory tell-tale
(paragraph 6.11.8.) shall be given if the ignition is switched off or the ignition key is withdrawn and the driver's door is opened, whether the lamps in
(paragraph 6.11.7.1.) are on or off, whilst the rear fog lamp switch is in the "on" position.
So it is not mandatory to have the rear fog light(s) turn off, as long as there is an additional warning that the light has been left on. That would explain why so many seem to drive around with the rear fogs on when not required.