Money doesn't grow off of trees.
well
nobody's claiming that it does
in theory, one of these referral links would have ads on their site, which they get paid for when they get traffic, so when you send people to their site, it earns them money, and as a result, earns you money
sometimes those services really aren't scams. they're always really inefficient, though, and you won't earn anything noteworthy until you've spent a stupid amount of time and effort on them