As someone who spends a LOT of time on the highway, I see a TON of people driving slow in the left lane and it drives me insane! The left lane is NOT for driving slow! The left lane is for passing! And it is baffling to me that these slow left-lane hogs don't even notice--or care!--that there are dozens of cars stacked up behind them. Do these idiots just never check their mirrors? Or do they just not care that they're slowing the pace of traffic? Either way, it's bad. And most states have left-lane laws that allow officers to write tickets to drivers who are going too slow in the left lane.
Here's the thing: it doesn't matter what the speed limit is. If you're driving in the left lane, and moving slower than the flow of traffic, or another driver wants to pass you, YOU MUST MOVE OVER. It doesn't matter that you're already doing 75; if someone wants to pass you doing 80, you have to move. It's law enforcement's job to write the speeders a ticket. It's YOUR job not to impede the flow of traffic. Don't believe me? Fine. Read this.
Here's an officer in Indiana who pulled over a slow driver and wrote a ticket because they were obstructing the flow of traffic. Not all heroes wear capes! Good on ya, Officer Wheeles!