Theory #5 - Social Justice
So this theory goes that some people take it upon themselves to keep the order in society. Many people go the legitimate route and actually become police officers or health inspectors. Others just enforce the rules unofficially in their every day lives.
On the roads this means that they will drive the speed limit regardless of the consequences to traffic conditions. I actually found a blog entry about this from a person in Canada. If they are driving in the left lane then anyone driving faster will just have to find a way around them if they are going to break the law. For them the entire road must abide by the speed limit... the left lane being no exception.
Strictly speaking, this is the law. People choosing the left lane don't have a free pass to drive over the speed limit under any conditions. However, the left lane has a reputation for a reason, that I argue has practical purposes. Take speeding out of the picture and the real reason people get annoyed is that there is no courtesy for people who want to get by. Not everyone should be forced to drive the speed limit on the road (some may choose to drive slower...) and for those that choose not to they should not block the driving style of others who may be more comfortable driving at higher speeds.
Bottom line here... leave the enforcement of the law to those who are paid to enforce it.