| It's hard to prove why a person braked. What if there was something small on the road that looked like it could puncture one's tires? What if there's a rainstorm and it's at night? Finding such an object after the fact would be difficult at best.
To be practical, the law must not penalize people who are simply trying to control their speed and deal with potential hazards.
I believe you should treat those behind you in traffic with courtesy, but the responsibility for their own safety must remain with each driver. | 
