I agree, that "places a higher demand" is wrong. Maybe "demands a better amplifier", but a lower impedance is not demanding anything, it is actually easing the flow of current.
But there in lies the problem. Think of it this way, a 0 Ohm speaker would actually be a short across the amp's terminals. So the lower the impedance presented to the amp the more current is going to flow, and the more unstable the components through which the electrons are moving will become.
Also an easy way to think about the parallel vs. serial connections would be to picture each speaker as a restriction to the flow. If the feed at the amp is initially divided and allowed to flow through each speaker beside one another; the flow is doubled because the restriction is halved. But if the flow has to go through one speaker and then the next; the flow will be cut in half because there is twice as much restriction.
