they aren't in any way evenly distributed along the political spectrum - the vast majority are some flavor liberal/left. Conservative viewpoints are dramatically underrepresented in general and more likely to be downmodded or not upmodded even without being in the batshit crazy demographic.
Anyway, you do have some point, but the reasons for these are objective. Simply put, conservative viewpoints tend to be more batshit crazy in general, and even if they're not, they're still wrong more often ("reality has a well-known liberal bias" and all that). Nevertheless, if you have a valid point, and if you can coherently articulate it, you will usually get upmodded even if it goes counter to the groupthink. It's just that some points are much harder to intelligently argue in favor of than the others.
There is a certain degree of inequality when it comes to the bar for getting upmodded. If you run with the groupthink, you can get easy upmods with just a single emotional statement with some invectives thrown in. If you're arguing against it, you have to be really persuasive. So if you look at all posts, there is a clear slant. But if you look at posts that actually contribute to the discussion, it's much more balanced.
Have a look at this old post of mine. It's about as anti-groupthink on Slashdot as you can get (note that this is back in 2009). And yet it doesn't have a single downmod, and one informative upmod. Why? Because I cited my sources and refrained from unsubstantiated attacks.