Most carrier promotions are not nearly that generation ($1,000 for an iPhone 7) plus, these inflated/subsidized trade offers typically have new line, higher price plan and/or term requirements. Additionally, customers may be sacrificing hundreds of dollars in other "bring/keep your phone" incentives in the process.
Again, I wouldn't call it "free" if there are obligations/strings attached and/or the buyer is giving up other incentives.