But what would Microsoft gain on that? I think they (unfortunately) make perfect business sense now. .
Quite simply, because in the long run, they will fail. Sooner or later, someone is going to put out a DirectX clone, and at some point, the industry will get behind it. Once that happens, they will lose control. It may take along time, but it will happen. Thus, they're better off trying to entrench the API further by offering help to port it. It won't impact their sales of Windows or XBox360 in the slightest, but it will do is increase developer mindshare.
You see it happening today with Internet Explorer. For a long time, they eschewed supporting open standards and insisted on adding proprietary semantics in their browser to enforce lock in, and it worked for awhile, when they had 90% of the browser market. They could do anything they wanted, and offering to hand over and commoditize features to competitors seemed like a lose-lose position. Then Firefox and WebKit provided viable, better, alternatives to IE, and their marketshare started slipping. Now with IE8, they have returned to the fold, but too late. Their position in weakened. The traditional powerstructure (W3C) has been made irrelevent, and the future of Web standards is now controlled mostly by Google, and anti-Microsoft contributors. It is likely that IE is on the path to obsolescence.
I know it seems utterly ridiculous and unthinkable, but ponder a scenario where compute shaders become a common programming model for lots of non-gaming tasks. There's no way data centers today are going to switch from Linux to Windows, and AMD/NVidia/Intel will surely want to sell solutions to these people, therefore, they have a vested interest in making sure minimally, a viable GPGPU/compute-shader driver architecture shows up on Linux, whether its OpenCL/LLVM or something else. Either way, it's not unthinkable that a separate industry push to run compute shader tasks on multicore or GPU via a standardized API would evolve into a render API as well. Microsoft would suddenly find themselves with no voice in a whole new market segment.
Anyway, if you look at Microsoft's history, only very few of their initiatives are profitable, many are cost centers aimed at heading off competitors in emerging markets. They have launched tons of APIs and products for which there's no real business model, except that they didn't want someone else to control it.