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.
but that does not help if the customer buys a 2nd 5$ charger and uses that e.g. at work. Then the touchscreen will behave less good and the customer will blame Apple...
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)