I bet the account (Apple ID user) who performs the brew builds has a full developer program ID. I looked at the terms of the developer program last night: Any user who logs in with their Apple ID can build applications, and use them on their own machine, but they can not distribute those. Once you get authenticated and get the full developer program (which either costs $99 per year, or can be free through a variety of programs), you can distribute apps either through Apple's App Store, or outside of it. I bet that brew counts as "outside of the app store".
I don't think the word "DRM" makes sense here. Membership in the Apple developer program does not imply that anyone has rights to the source code or compiled application. I see it more as a way to authenticate and identify developers, and preventing unauthenticated developers from distributing software. Whether that's a good or a bad thing is in the eye of the beholder, and depends heavily on the use case.