When I first started looking into the issue of TrueType fonts, the Microsoft Core Fonts were distributed under a very liberal licence, which, among others, stated the following:
Reproduction and Distribution. You may reproduce and distribute an unlimited number of copies of the SOFTWARE PRODUCT; provided that each copy shall be a true and complete copy, including all copyright and trademark notices, and shall be accompanied by a copy of this EULA. Copies of the SOFTWARE PRODUCT may not be distributed for profit either on a standalone basis or included as part of your own product.
This licence would have made those fonts suitable for inclusion in a GNU/Linux distribution (for example in the non-Free part of Debian).
A few months later, TrueType fonts had become a viable alternative to Type 1 on Free systems. At some point, however, the Web Fonts licence had been changed to say:
WITHOUT LIMITING THE FOREGOING, COPYING OR REPRODUCTION OF THE SOFTWARE TO ANY OTHER SERVER OR LOCATION FOR FURTHER REPRODUCTION OR REDISTRIBUTION IS EXPRESSLY PROHIBITED.
The fonts packaged as ZIP archives have also ceased being updated, and all the newer fonts have been distributed in a format that can only be unarchived on Windows machines.
Microsoft, TrueType, Windows, Linux, and possibly others are trademarks or registered trademarks of their respective owners.