I would refine your argument a little. The Free Software movement does not maintain that "software should be free because it's wrong to restrict developers' abi

I would refine your argument a little. The Free Software movement does not maintain that "software should be free because it's wrong to restrict developers' ability to use, copy, modify, and distribute it". It maintains that 'software should be free because it's wrong to restrict users' ability to use, copy, modify and distribute it'. There is a subtle distinction. Open Source is generally in favour of developers' freedom, Free Software is generally in favour of users' freedom. These come into conflict when, for example, developers want to lock their users into SaaS walled gardens. Open Source advocates are generally in favour of the developers' freedom to do this. Free Software advocates are generally in favour of the users' freedom to not be locked in (see the AGPL for example).