In addition to all the obvious reasons, tech blogger Jillian York recently noted that some web filters use comments, and links in comments, to categorize sites as pornography. This means even a blogger who diligently refrains from any, er, PhysioProfisms, can be classed as offensive and filtered based on comments alone. York explains how recently her blog was blocked by Websense:
My assumption was that their automated system was based on keywords, and that my blogging about Helmi Noman's paper ("Sex, Social Mores, and Keyword Filtering: Microsoft Bing in the 'Arabian Countries'") had caused it; after all, it caused "Arab sex" to be the #1 search term for my blog. Turns out, that wasn't the case at all. In fact, what happened was significantly more chilling. . .
York obtained an explanation from Websense' PR team, who seem oddly helpful as web PR teams go:
. . . the problem seems to come from the comments, not the posts. Indeed, you appear to be the victim of comment spam (which often contains pornographic links or links to malware).
Look at the comments after this post: http://jilliancyork.com/2008/09/11/blog-strike-for-mohammed-erraji/. The last comment has pornographic links and the one preceding it has links to pharmacy spam, which often leads to malware. This is just one post that we looked at. You may have more.
York clarifies that yes, third party spam comments caused her blog to be filtered as porn - which means that anyone could theoretically block someone else's blog, merely by spamming it with links to porn sites and pharmacies. Granted, it would only work for filtering software that looks at comments; in York's case, the software made her blog inaccessible to foreign visitors from some countries the use the filtration software - specifically, middle eastern countries. That may not be a big deal to you. On the other hand, maybe you don't even know your blog is blocked in other countries - maybe you've never had reason to know. Filtering companies don't necessarily have an incentive to be fair or careful, especially when it comes to smaller bloggers.
We have no details on how many filtering systems would be affected by this kind of thing, how many viewers would be impacted, which countries, or what the threshold level of spam is before your site is deemed to be filterable porn. The really bizarre thing is that York is hardly uncontroversial - she writes for Al Jazeera's English language website and is a free expression activist who studies government censorship in the middle east. (And yes, the top search term for her blog is apparently "Arab sex.") So the fact that she was blocked there isn't that surprising - but that it was for some silly comment spam is just too ironic. Sigh. And we thought all comment spam did was annoy. . .