No, it's a reasonable question. I wondered myself a while ago.
Here's why. From Wikipedia: here (http://en.wikipedia.org/wiki/Ground_%28electricity%29#Circuit_ground_versus_earth).
[i]Circuit ground versus earth
In an electrical circuit operating at signal voltages (usually less than 50 V or so), a common return path that is the zero voltage reference level for the equipment or system. Voltage is a differential quantity, which appears between two points having some electrical potentials. In order to deal only with a voltage (an electrical potential) of a single point, the second point has to be connected to a reference point (ground) having usually zero voltage. This signal ground may or may not actually be connected to a power ground. A system where the system ground is not actually connected to earth is often referred to as a floating ground.[/i]
Oh, and I'd say click on the non-conductor link anyway, but it's a really lame joke, so don't bother.