Bad power really messes with the touch screen. Capacitive touch screens need to sense pico-Coulombs of charger, and this is really hard when the power supply (and the display) create tons of noise. This is a bigger problem as phones get thinner because it's harder to shield the touch screen. For details, see http://www.cypress.com/touch/Noise_Wars_Projected_Capacitanc...

My totally hypothetical theory is that Steve Jobs tried a prototype charger, the touch screen didn't work well, and he said to do whatever is necessary to make sure that didn't happen again.

(edit: more info)