Just to let you know David, some companies do release "Beta Demos"
Jowood, for example, released the Soldner Beta Demo
http://soldner.jowood.com/?RubrikIdentifier=726&lang=en
Look down the page to where it says "US Beta Demo"
Now to the most extent you are right however, as most companies wont publicly release beta demos (unless they are apart of the MMORPG world, but they call that "Open testing") and most of these that are found have been leaked by a beta tester