Too much pressure can mess up the touch screen. I went to a Walmart yesterday and on the DS, someone had pressed the stylus to the screen really hard causing a noticable imprint. The touchscreen was inaccurate in detecting where you put the stylus. It pisses me off, because there's no way to protect demo systems from assholes or children that don't know what they're doing.
Maybe they could put the DS demo unit behind the counter, playable upon request. I mean, I couldn't imagine them demoing a Pocket PC in Walmart for any customer to try.