It is not that Docker Desktop is not free on specific operating systems. Docker Desktop is not free for large companies. Doesn’t matter where they run it. Docker and Docker Desktop is not the same thing. Docker CE (community Edition) is part of Docker Desktop running in the virtual machine and that is free.
*Docker Desktop is free to use, as part of the Docker Personal subscription, for individuals, non-commercial open source developers, students and educators, and small businesses of less than 250 employees AND less than $10 million in revenue. Commercial use of Docker Desktop at a company of more than 250 employees OR more than $10 million in annual revenue requires a paid subscription (Pro, Team, or Business) to use Docker Desktop. While the effective date of these terms is August 31, 2021, there is a grace period until January 31, 2022 for those that require a paid subscription to use Docker Desktop.
You can also run Docker CE on Linux or just use WSL2 and install Docker inside that or in any supported Linux virtualmachine.