Saying something is dumb is probably not very helpful. It was touched on briefly, but not in a way likely to help someone learn why.
Throttle spacers work in carbureted engines by changing the length of the fuel runner and allowing for more thorough atomization of the fuel and giving it more time to warm up (even more atomization). Even in a carbureted fuel system, the spacers are not long enough to make much of a change. Say the A/F mixture is in the runner for 12 inches, a BIG spacer would be all of an inch and change the length of the runner by only 8%. Add to that knowledge the fact that the charge is in the runners for mere fractions of a second, and you're really not talking about much in terms of heat being transferred. You're probably not getting much of an effect from the additional length either for that matter...
Meanwhile, despite minimal (if any) gains in a carbureted engine, they don't work in a modern port injected engine at all because the throttle only controls air flow and fuel enters the charge well after the throttle body. Since there's no fuel in the air at the point of the throttle body in a fuel injected engine, there is no effect on the charge. If anything, it would lessen the turbulence in the air and reduce the atomization (again, minimal to no effect).