To be fair, a pyramid scheme is not necessarily a scam if it is acknowledged to be a pyramid scheme. Early "investors" will indeed get double their money back, at the expense of the later investors.
Think of it as speculation: "I wager that I will be in the first half of investors." Or really earlier than that, depending on what kind of a cut the "scammer" takes.
Of course, that doesn't make it a smart investment.
Yeah, it's possible to make money off scams, but I think it's somewhat immoral since you're encouraging the scammer to scam and directly and/or indirectly encouraging others to be scammed. But it's not the worst immoral act, so I don't really care if others do it. I just don't like to participate myself. Although I must admit, sometimes it is tempting to get in while the going is good.
Must... resist...