I work in information security for a company that uses content filtering, let me explain the standard reasoning (with the disclaimer that I don't speak for my company or yours, etc).
Your work computer and Internet access is governed by a policy that all employees have to sign. Among other things, this policy says that your time at work is supposed to be used doing work-related things. The only way to enforce this policy is to filter out things that are not business-related. This policy at many companies is usually pretty lax and ad-hoc, really only blocking categories of sites, not individual sites themselves. The categories usually include pornography/extreme content, games, etc.
Other categories are blocked due to the fear of data leakage. Google Docs/Dropbox is common for this. The last thing you want your employees doing is putting sensitive information onto the Internet where competitors or ill-wishers can view them. The goal here is to make it more difficult to share, not impossible (knowing that email/removable media is still an option).
We want you to do your work. We understand there are times when you need Internet access. But no, we don't trust you, because you're on Hacker News right now (as am I). If the world were a better place, we could trust the security of the company to the users/employees. You and I both know this isn't true. While you're at work, your company owns your time under the terms of your contract. It's not censorship because private organizations have no responsibility to ensure your freedoms.
TL;DR try complaining to your CIO that you can't get to iheartchaos.com. S/He'll tell you to get back to work.
There's a special place in hell for business owners who inflict Websense or any other method of censoring content on their own employees.
If you don't trust me to use it, don't give me a computer, or an internet connection.