since many are sold as back up power units, it is not logical to have a idle down feature, as so many things have need of a trickle of power to signal the unit to work, and to have some thing start and it voltage is low and frequency is off it is really hard on the equipment, it may burn up a few thousand dollar furnace, fairly easy,
(you do not see auto start systems hardly any more, my folks had a generator after ww2 that you would flip a switch in the house and about 15 seconds later the generator would auto start and the light would come on, and when the light was shut off it shut down when the need for power was not needed),
the fuel used is not that much more just to idle the engine at speed, as the work to keep the engine spinning at 1800 or even 3600 is not that much more than at 1000 rpm, so most do not worry about it,
If I was to use my welder as a generator I would not use the idle down feature as it make things work so hard starting up, (I hardly even use it on the welder as it makes getting the arc started harder),