Perhaps I chose the wrong word. By "scam" I meant that there is no reason you'd even need the OGL or GSL to produce content for 3e or 4e. But many, many people used it because they were unaware that they didn't need it. As long as no patents or copyrights of another company is used, material can be produced that "works with" another companies game. No need for a license. The d20 companies signed a license to do something they already had the power to do.
I agree that no one *has* to agree to the terms of the OGL/GSL in order to produce products compatible with D&D. Publishers choose to do it for a number of reasons, including:
Legal: By agreeing to the terms of the licenses, you place yourself in a safe harbor that
mostly shields you from the kinds of lawsuits you might face if you didn't use it. Remember the lawsuits by TSR back in the 80s against companies that produced compatible products? Remember that the legal bills of those companies forced them into submission by the behemoth that was TSR? That's a fairly significant risk to take when you don't have to (and it costs you nothing).
Distribution: Agreeing to the licenses increases the likelihood of getting carried by the major distributors. That means you have a chance of showing up on the shelves of your favorite local gaming store. For digital distribution, it means you're going to get placed with other products of the same system.
Consumer Acceptance: By and large, consumers accept the OGL/GSL. That means they understand that products that carry the standard notices required by these licenses will work for their games. Without that, you get to build up trust from zero and hope enough gamers purchase your products. That's not easy, cheap, fast, or painless--and most of the companies producing products today grew their fanbase from their success with the OGL/d20 and now GSL.
So, as long as you're prone to taking extraordinary risks, willing to invest a lot of capital in getting your product known (and perhaps a boatload more for defending every assumption about what's permitted and what's not under copyright and trademark law), and slowly building up a product line from the ground floor, you can go without the benefits that the OGL/GSL provide.