I believe this is because if you have it configured to not cache on the server then it also does not minify. The  idea being that you should always have server caching enabled unless you are working on the design in which case it might help to be able to read the CSS easier. Minifying is a somewhat expensive process so if caching is turned off we are doing this expensive processing on every request, so it makes sense to not minify when not caching to avoid having this expensive process on every request. If you have it set to cache on the server it still would not cache for your own requests if you have the cookie that says don't cache, but it would cache and minify for other visitors.

Best,

Joe