Surely it's the ones doing 80mph that are the hazzard
Look at traffic as a fluid: A stream is calm when the water flows at the same speed and direction, when there is nothing to get in the way. Put a rock into the stream and now water has to move around it, causing flow patterns that collide. Traffic can be looked at as a fluid in the same way. If you want it to flow without friction, it all needs to go the same speed in one direction. Accidents and traffic slow-downs are caused when people merge and people have to slow down unexpectedly, which is going to happen when you're going 30mph under the prevailing traffic speed.
Yes, the person going 55 is following the law, but the law was written in a faraway room, a long time ago, by people not driving on the road in that instant. Sometimes in the instant, following the law is not the safe thing to do. It's the same reason it's not safe to go 25 in a 55, and that trucks are required to put their flashers on if they're going under a certain speed. Because they hare a hazard if they are going too slow.
This is not correct. Many states in the US do not allow the use of hazard lights while moving.
You'll have people passing you on the left and behind you at +30mph. The cars behind you in your lane will form a queue of waiting to pass you, and they'll be getting into the right lane needing to accelerate from 55mph to 80mph (hard for a lot of cars). This is going to cause the cars in the fast lane to brake, which will cascade through the lane and cause a traffic slowdown. If someone isn't paying attention, it's going to result in a rear end collision and an hours-long traffic jam for hundreds of people.
This is one of my worries with self driving cars: They'll follow the letter of the law even when it's not practically safe.