To the best of my knowledge, all components of the MiKTeX software are freely redistributable (libre, that is, not necessarily gratis), within the Free Software Foundation's definition and the Debian Free Software Guidelines.
That said, the MiKTeX software has neither a single copyright holder nor a single license covering its entire contents, since it is a collection of many independent components. Therefore, you may use the MiKTeX software only if you comply with the requirements placed thereon by the owners of the respective components.
To most easily learn these requirements, I suggest checking the source code.
Christian Schenk