According to one of the comments quoting an excerpt from the license for Microsoft's Web Application Installer, MS does not agree that they are "distributing" open source.
THIRD PARTY SOFTWARE. This software enables you to obtain software applications from other sources. Those applications are offered and distributed by third parties under their own license terms. Microsoft is not distributing those applications to you, but instead, as a convenience, enables you to use this software to obtain those applications directly from the application providers. By using the software, you acknowledge and agree that you are obtaining the applications directly from the third party providers and under separate license terms, and that it is your responsibility to locate, understand and comply with those license terms."
THIRD PARTY SOFTWARE. This software enables you to obtain software applications from other sources. Those applications are offered and distributed by third parties under their own license terms. Microsoft is not distributing those applications to you, but instead, as a convenience, enables you to use this software to obtain those applications directly from the application providers. By using the software, you acknowledge and agree that you are obtaining the applications directly from the third party providers and under separate license terms, and that it is your responsibility to locate, understand and comply with those license terms."
Phew... That was a close one. :P