The "ensure they use it" is the part that is uncertain, and thus the 'scam' aspect. Amazon (and other vendors) are betting that a significant percentage of end users will not actually use it, and the seller will get to keep the entire balance as profit.
According to some not-very-thorough queries, somewhere from 10–19% of gift cards are never redeemed in the USA, which equates to tens of billions of dollars. Unreal. If even half of that is true, that's a tremendous amount of charity to corporations.