The Service provides users of Ubuntu with a free subscription to use the Service on up to five physical Ubuntu systems with unlimited number of VMs or containers for the eligible Ubuntu LTS versions as designated by Canonical.
https://canonical.com/legal/ubuntu-pro/personal
Why would you assume to own software if you don't even pay to use it?
You can do a bullshit fork and then sell it, but if you owned it you could sell the exact same code with zero changes
To prove your point you'd need that example: an open source program that you can sell for profit with zero changes.