What you just described is a system that mistreats your workers. Those temporary hires, if they lose their jobs, they can’t put food on the table. But if the company loses a temporary worker, it’s not going to be troubled, they’re just going to go hire another person.

That all being said, if you’re working under contract and your company has robust protections for retaliation by employers, some of the risk of telling them in advance goes away. That’s great, but there’s still some remaining risk. Many bosses will be vengeful, bitter, and they may sabotage your work however they can for the last few weeks or months. And you won’t be able to stop them, because you’re leaving, so even if you filed an internal complaint, it wouldn’t go anywhere.