The role of a contractor is to act as temporary, supplemental workforce to augment staffing levels when there is a gap. Obviously, when there is a workforce reduction these contractors are the first to go. The contractors understand these risks when they accept the position and they are compensated for it. A company such as GM will typically pay the sponsoring company $90/hr to $125/hr per contractor. Obviously the contractor themselves does not receive that full amount of money, but they do walk away with a hefty hourly wage.
Most contractors have already been notified their employment with GM has been terminated. There are likely some areas of the company where contractors will remain beyond the end of 2018. Some of these individuals will simply be asked to leave at the end of their existing contract.
Reposted this from @WtKL5ta-3dhz . Thought the OP had a point.