I don't think anyone was selling demos commercially or trying to pass off the creative ideas as their own work. With this in mind, we should set aside ideas of plagiarism, copyright, etc. It was a showcase of technical prowess/creativity. People knew what Death Dealer looked like & if they saw it pop up in a demo, they wouldn't think the demogroup was passing it on as their original idea (I would assert this was a given). As such, it was meant to be a reference. People thought they knew the limitation of their computer. They would play Lemmings, or whatever, and think that's as good as the graphics on the Amiga can get. The point of the demo was to blow those conceptions away.

The creative part in a demo wasn't the the art itself, the subject, the composition, etc., no, it was representing something thought impossible. Eventually, kinda like how photography changed painters' relationship with realistic representation, more powerful tech did the same with these types of demos, so the medium moved on.