Can we apply that same logic, then, equally to the Unions?
Insofar as they are considered distinct “persons” under law, then yes.
Your first mistake is assuming there is any consistent logic that Sotomayor feels obligated to apply ... :-)
And if you asked her that question, you would probably get an argument that unions are voluntary (!) associations of working-class individuals, thus speaking for those who "don't have a voice in this oppressive society", while corporations have extraordinary hiring-and-firing power over their employees which means they don't represent working-class people and are just an unfair amplification of their greed-motivated management.