These fonts were originally released by Microsoft as executable installers, and their license only allows "complete copies" to be distributed:
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.
Ubuntu's package doesn't actually contain a copy of the fonts, but rather a script that downloads the original Windows executables from a third-party archive. The script then extracts and installs the fonts without actually running the executables.
It's a convoluted arrangement, but that's what happens when software is released with restrictive licensing.