In addition to what mankyd said, this printing method ties precision to the pixel density of the screen. Low resolution screens like the one you linked give lower quality prints. You would really need to step up to a higher-resolution small tablet or large phone (something like [1]) to get a package that works well for the average consumer. That's going to add at least $50 to the BOM and make overall development trickier.

[1] http://www.aliexpress.com/item/12/32579900233.html

They'd still need software to control said hardware. We can assume such software would be mobile, based on what they're currently offering.

Then they'd need a way to pair the phone with the controller, or otherwise get the data over. This complicates things for the owners. At that point, you might as well just use the phone itself.

Mind you that this limits the versatility of what they're selling - you can only print something as big as your phone - but they aren't aiming at the high-end market with this.