I'm not sure what the psychology behind this is, but it seems to be a fairly common reaction. I noticed it today on MJ Ray's blog where - as far as I can tell - he's upset that the Conservative party, whom he doesn't support, are advocating Co-ops, which he does support. (I know that's an oversimplification of mjr's position, but hopefully it's not an outright misrepresentation of it. UPDATE: Ok, yes, I did have his position wrong - see the comments).
I can only presume that it's the same thing that ensures that every time Microsoft does something pro Open Source / Free Software, there'll be a furious post on Groklaw about it.
Or why people who for years have hated the way the MPAA and RIAA treat their creative talent are now annoyed that the Writers Guild of America is standing up to them, because the WGA is a union and unions are baaaad.
I suspect that the temptation to react this way is similar to the temptation to flip the bozo bit on people. Instead of deciding that the person is an idiot and therefore can't possibly have anything useful to contribute - and so can be ignored - we decide that the person (or group) is evil and anything they do must be outright harmful and must be opposed.
In reality not even a single person is ever completely useless or completely "evil"; everyone will at some point have an idea that's worth considering or an opinion you agree with. And that goes double if there's more than one person involved - any large organization, in particular, will probably have some subgroups that you agree with more often than not. Recognize when you have common ground even with people you normally disagree with. That's a vindication of your ideas, not something to be upset about, surely?