It undermines their business in that it benefits a competitor to their Windows Phone offerings in the form of the Neo900. (and potentially gives other competitors access to some of the things that made the N900 good)
Not to mention any 3rd party code they used and cant release for that reason. And any proprietary algorithms/info they cant (or wont) reveal (e.g. the secret algorithms used for the audio stuff or the secrets of how to talk to the cellular modem or the secrets of how various bits of 3rd party hardware work or the secrets of how the N900 talks to Microsoft email servers (details that Microsoft doesn't want to share for obvious reasons) There may even be things they cant release for legal reasons (e.g. the wl1251-cal daemon used to pass configuration information to the WiFi chip, releasing the code for that might make it possible to configure the WiFi chip to use different country settings than they should be using which would then cause the chip to act in ways that are illegal)