I don’t think it’s a question of intelligence. It is just a matter of oversight. Consider that a person goes down to his basement one morning. There on the floor is an inch of water spreading all over the place. Panic! It must be the water heater!!! Shut off the water! Drain the tank!.
The latter action relieves the problem until a plumber can arrive to do what is necessary. In the New York area at least, plumbers do not come immediately on call. Some of them may not arrive till the next day. Meanwhile, the precaution with the gas valve being turned off is totally forgotten.
Prime requisites of design for any piece of equipment–and most especially for something that will be in the hands of untrained individuals such as homeowners is that first the product functions safely under normal use as well as under all conditions of foreseeable abuse. And secondly, when the product fails, it should do so in a safe manner (such as coming to a complete stop, or the gas turning itself off, etc.). In the case of putting the cutoff valves into both lines of a water heater, you violate the second requisite: failure of the heater leaves it in an unsafe mode.