No, but the cost you personally pay for your STC is a _very_ small fraction of what the STC owner had to pay to go through the process to convince others that their product is safe, and they also took a risk that the STC might not ever be permitted.
Extreme case in point: The Cessna 182 (certain models, at least) has a maximum gross weight of 2950 pounds. A company sells an STC to increase that to 3100 pounds without any hardware modification. A piece of paper that allows me to carry additional weight, as long as I'm willing to pay $750 for it. Quite an expensive piece of paper, no? Well, maybe not, since the company that owns the STC put in a lot of time and money to get FAA approval for that STC, and since they succeeded they are due their payback. If things worked according to your view of "it's already been shown to be safe, so we shouldn't have to pay ourselves" -- do you think that STC would have ever been researched, knowing there could be no return on investment?