Hmm.. it's a resistive touchscreen, and skin contact with the bottom of the Arduino would probably give you a connection to the device's ground plane. Skin resistance varies dramatically, but the Drawdio and the MaKey MaKey rely on the fact that some small amount of current can flow from one part of the body to another. From there, my reasoning gets very speculative though..
Some people do have trouble getting a good reading with their fingertips. Do you get the same results when using a fingernail or a stylus (the non-point end of a ball-point pen, for instance)?
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.