You could look at it as both. It is the law that you are not allowed to go over the posted speed limit. Most police officers give you a little leeway (maybe 5 MPH over) to account for the inaccuracy of your speedometer and/or the equipment they are using to measure your speed, but they don't have to.
It can also be seen as a guideline as to what the max speed limit is under ideal conditions. Any adverse conditions, you do not have to go that speed, and should go much slower. In fact, you can still be ticketed for doing the speed limit with bad road conditions (it's called Basic Speed Law in many places).
As far as creating a danger, I don't see how that is happening. If I am doing the speed limit and there are 5 cars behind me that want to go faster than the speed limit, how am I creating a danger? If they are following too close to me, they are the ones creating the danger, not me. I am not forcing anyone to follow too closely or to pass in an unsafe manner. It is not my fault these people are in such a big hurry to get where ever they are going (their own funeral maybe?).
