Why does MM's lead reduce damage penalty when none is dealt?

On a rebel's turn, they call up to a 5 assists (4 teammates, plus summoned rebel). If the rebel who is calling assists does no damage (MM basic, or maybe target had foresight), the assists suffer a 45% percent penalty, instead of the normal 90%.

I think you misunderstand, that if an assist call did no damage it's damage penalty was reduced. Which would make no sense, as you point out.

Note- I've defined rebels as Rebel Fighters + MM.