So they can leave, and make their own fork of the project.

When they joined the CNCF, they agreed to give control of the project to the CNCF, and a big reason for that is to give users of the project assurance that the license won't change if the vendor decides it doesn't want the project to be open source anymore. If a company can change its mind and claw back a project, that undermines the purpose of the CNCF.

Also, membership in CNCF is distinct from having a project contributed to CNCF. As far as I know, there is no requirement that you be a member of the CNCF to submit a project to the organization. And I don't think you have to contribute a project in order to be get membership.