Google down-ranks unnatural links

I mentioned that this blog is blocked by some filtering software. Now I get this email, about Google's censoring policies:

Hi George,

We would like to discuss an important matter with you regarding your site.

It has come to our attention that Google has penalized our site due to our backlink profile. Unfortunately, a link to our site on your website may be considered unnatural by Google and may be partially responsible for our penalization.

Because of this, we request that you remove the link to our site that is located here and says: ...

Apparently this firm posted some spam in my blog comments a couple of years ago, and now Google is penalizing them for it. So now they want me to remove the comments.

Perhaps I should be happy that Google is discouraging spam, but who appointed Google the policeman of the web? I am using Google to filter the spam, so if some got thru, I blame Google. (Occasionally a legitimate comment gets held up by mistake.)

The questionable practice here is that Google penalizes sites for what it considers unnatural links. Apparently I could punish a site I don't like by putting a bunch of unnatural links to it on my blog. They would be left begging me to remove the links, as the above letter does. I could refuse.

Google complains a lot about censorship in China and elsewhere, but it does its own form of censorship when it down-ranks web sites out of disapproval of the links to those sites.