But there's a barrier for posting correction posts: they are often awarded with negative reputation.

I don't find this to be generally true, but maybe it depends on the tone of the answer. Correction posts which insult the author or disparage his ability to program seem less well accepted than those which simply point out the mistake. I would construe this as a feature of the PM system rather than a bug though, because civility can be more important than content.

And I also think that a chance at receiving negative votes should not prevent one from posting content that one thinks appropriate, just as the chance of receiving upvotes should not be the only reason one posts (wow, lookee here, full circle to the OP ;-). Personally I've learned a great deal from answers which correct mistakes or misunderstandings in nodes that I wrote, and I think it'd be a shame if people held back on constructive criticism just because they think it may be unpopular.

But claiming gravity doesn't exist isn't a "point of view". It's plain wrong.

Unless gravity really does not exist in your frame of reference because you're a CGI script. :-)