Microsoft's old Windows Mobile platform, which had no restrictions at all, enjoyed a substantial homebrew developer community. That community was largely displaced by Windows Phone, due to the Marketplace restrictions, leaving the (rare) fans of that platform disappointed and looking elsewhere for their smartphone needs. In backing ChevronWP7 and re-engaging with this homebrew community, Microsoft is welcoming these long-time supporters back into the fold.
There's no doubt that Microsoft's approach to the chevron team has been unlike other hardware manufacturers. On its availability early in 2011, the Windows Phone didn't go for the draconian "shut it down" policy that others have employed (notably Sony on the Playstation Portable), but worked with the team to come up with something that satisfies everyone involved.
And that says a bit more about Microsoft - it says that they are listening to the developer community and are happy to work on compromises. Given that application ecosystems are going to be one of the key battlegrounds to establish Windows Phone, Chevron support might be one of the biggest adverts possible to the developer community.