*The justification comes from politics itself, not the philosophy.
In my opinion, philosophy is about overcoming epistemological obstacles ("conditions of possibility" for a politics), not the politics itself. And in this regard, I find many poststructuralist theories particularly illuminating because they overcome traditional Western metaphysics, particularly the division between subject and object, empiricism, etc. Even though I said philosophy does not EQUAL politics itself, it can also be implicated in imperialist projects, for example, positivist empiricism and its relation to statistical representation, colonial authority and imperialist subject constitution. I hope this answers your question, if not, or if you'd like to discuss more, feel free to PM me. Also let me know if I'm not being clear. As I've said many times before, I'm no expert, so I might be misusing language in my novice attempts to explain what seems so attractive about these theories.
I don't think you can "justify" anything with it, ie. Foucault's method is just a way of overcoming epistemological obstacles and diagnosing current queries in order to find the right question that has a "critical purchase" on our social situation. Foucault just happened to be tending toward the Left. Spivak had an illuminating quote, forgot where I read it, but she basically said look, Derrida isn't supposed to do anything, he doesn't have to be a feminist and he doesn't have to be anti-imperialist. But you can use his philosophy in order to interrogate and expand those positions. Similarly, Derrida once said you could probably have deconstructivist conservatives.
i actually like foucault. i took a course of critical theory at school and most of it did not convince me. what do you find so attractive about it? dont you think you can justify virtually anything with it?
