You're not taking anything away when building open source software into proprietary systems
From Stallamn's view, I'm pretty sure you're not taking anything away by restricting the creation of more proprietary software. Proprietary software pretty much doesn't exist for him.
To understand his point, you really have to apply all of the moral frames relating to 'freedom' to software licensing. In that view, to license software without distributing source is akin to licensing software with a clause to give up your first born.
From Stallamn's view, I'm pretty sure you're not taking anything away by restricting the creation of more proprietary software. Proprietary software pretty much doesn't exist for him.
To understand his point, you really have to apply all of the moral frames relating to 'freedom' to software licensing. In that view, to license software without distributing source is akin to licensing software with a clause to give up your first born.