Originally, Microsoft Real Time Communications (RTC) Server was a component of Windows .NET Server (Win.NET Server) 2003 and thus built into the OS. If you go back through the early betas of Win.NET Server, you’ll see the SIP Services component in the Control Panel.
Microsoft revised its plans for the delivery of the RTC Server and has decided to package it independently. In my opinion, this change in plans wasn’t made for revenue-generating purposes. At the time of this writing, the licensing wasn’t set. However, chances are that you’ll have to use the Client Access License (CAL) mode to pay for access to RTC services, so whether the server is part of the OS is irrelevant. I think this repackaging gives potential implementers more flexibility in terms of how and when they can build an RTC-based environment.