All of the Microsoft technologies which have been open sourced over the last couple months have been under MIT which is really strange actually as Mono is licensed under LGPL2 which is actually more restrictive!
Wouldn't licensing under GPL limit their flexibility with respect to external contributions? They wouldn't be able to use that in proprietary product, or move the project's development back to a closed model.
Depends. Some companies use GPL with copyright assignment to control the project: you can use it as a wholly open source product, but the company has the right to do future development in a closed source way, embed it in proprietary products, etc.
I don't know if that's likely with a build tool, but it happens elsewhere.
Fancy that, how times have changed.