I have been using FreeBSD since 1999, a great OS and i love it, at work i am a Linux system administrator.

One of the huge reasons people use FreeBSD is simply licensing. If you dont want to release any source code simply build your custom app on FreeBSD and only include BSD licensed software. Makes being proprietary simple.

Note, this does not mean any companies that do this do not give back to the project, they do in the way of code commits and sometimes donations.