Small companies can quickly and easily leverage open source software to save money, but in an Enterprise environment, once you've selected your platform, be it MS or Open, deviating from that platform can be costly. If you've got a standard MS desktop image rolled out across the hospital, are using System Center Operations Manager to manage them, and have business processes tightly integrated with Sharepoint, implementing open source now becomes a multi million dollar migration effort, and breaks the numerous benefits available from a homogeneous environment.
Management dictating a Microsoft (or any other) ecosystem as policy, and requiring IT to support that policy is in line with Enterprise IT best practices.