I don't think the software can be pointed at as it can be configured to allow guest posts.

However forum spam is a problem and without registration we would be overrun with posts about casinos.

Another major reason, is people here take their time to answer people's questions, and if people are registered they can have an email message when someone replies to their post - meaning they are more likely to read the response. Without the email reminder they would have to bookmark their post and check back every now and again to see if a reply has been posted, and if they forget to do that then it's just a waste of time answering their question.