I think it's because Adobe doesn't really have any competitors in its marketspace (aside from Flash).
Adobe's bread and butter is still the Creative Suite. It doesn't view GIMP, Inkspace, etc. as a big enough threat, so why give it away for free?
Microsoft on the other hand, is competing with Java for most developer mindshare (in the "conventional" space) and RoR, Django, PHP (in the "unconventional" space). Microsoft also knows that the GNU C++ compiler isn't half-bad, and additionally with the license change in Qt, it's even easier.
Microsoft wants to nurture small microsoft shops in case they become big microsoft shops one day. It's a way to keep the ecosystem thriving!
Once Adobe has a competitor that can compete on technical merit (and not just, "use this b/c it's free!"), then maybe Adobe can consider something similar.
It's very, very, hard to compete on price. If that's your business strategy, you're going to fail in the long-run. You have to have some amount of technical innovation in your product.
If MS's Silverlight catches on, Adobe and Flash will have some competition. But it seems most designers I know have some sort of serious pathological dislike of Microsoft so it seems rather unlikely.
Also Adobe products are sold to a subset of a company where MS tends to endup on everyones desk. So getting a startup hooked is far more valueble for MS than it is for Adobe.
Microsoft benefits immensely from having programmers building things that run on Microsoft technology. It's the consequences of the tool use that are important, not the tool use itself. Adobe, conversely, doesn't gain anything from having images in the wild that were drawn by Adobe tools. Adobe is a tool company, so they charge for tools.
There's a good Joel Spolsky quote saying that the only reason Microsoft doesn't give its development tools away for free is to avoid accidentally cutting off the air to the 3rd party vendors who build and sell add-ons. There needs to be a little money changing hands in that space so that companies are used to spending money on tools, and therefore are willing to pay for things like ReSharper, Ants Profiler and CodeSmith.
