Interesting to listen to Scott Hanselman and Richard Campbell talking over the rationale for open sourcing Microsoft's ASP.net MVC framework on Hanselminutes show #175.
Part of the answer was a general desire to nudge Microsoft further towards embracing open source: "Why wouldn't you?". Which is admirable.
Partly it is a desire to open up the innovation envelope: Scott talked about his experience releasing TweetSandwich, and then seeing the unexpected derivative applications developed using the source as the base. Designing a framework is a daunting task. By definition, most of the framework's possible uses are not known ahead of time.
Take a listen to The Java Posse #241 which also came out this week, where they discuss the challenges of design as it applies to frameworks. One of the great concepts they talk about is how the best frameworks invariably have well designed escape hatches, to make sure you can overcome that typical problem of '..but the demo worked so well!'
Personally, I think having access to the source code of the framework is one of the most effective 'escape hatches' you can have.
Even if you never plan to fork or modify the framework, the ability to dive in and examine the source when things are not quite working as expected is really the difference between a framework you can work with, and a framework that will be discarded after a couple of projects. It is one of the great things about rails: often the documentation comes up short, but when you look at the api, the source code is but a click away!