I think companies could mandate what OSes their servers use and standardize some of the software that the development team uses, but to require an OS is pushing it. I understand that many non-technical companies do this, however operating systems tend to be a personal preference. Some people are more efficient on Windows, other on a Mac and others on their Linux distro of choice. If you have to use one OS for work and another at home it just creates a productivity conflict. Big "Take this OS and Shove It" from here.