I was surprised about this translation! Could it also mean anything like "not by force"?
No, "by force" would usually be "con la forza" (with/through force). "Per forza" is an idiom meaning that's how it has to be (per cause di forza maggiore, as it's often formally stated, "due to causes beyond our control").
Italian idioms...ya gotta love 'em, but first...ya gotta learn them!