That sort of thing is a reaction, by developers, to Apple's unwillingness to add demos (and consumer's unwillingness to shell out when there's no demo). Calling

That sort of thing is a reaction, by developers, to Apple's unwillingness to add demos (and consumer's unwillingness to shell out when there's no demo). Calling it a "pen" is probably not the best way of marketing it, but basically you're buying the full version. The free install is a trial. If Apple added demoing all of that would vanish pretty quickly, I suspect.