The Nords rule The Reach, simple. There is no such thing as legitimate rule, Tamriel belongs to the strong. If the Forsworn want it back they can try and take it, they clearly lack the strength though and resort to murdering civilians travelling on the roads.
This is a form of Gurilla Warfare. They hit, and they run. Its not cowardice, its how a small army fights a big army. Always has been.
American Revolution, we had a highy underpowered millitary, and the British had the best millitary in the world at the time. There were many things that sided in our favor however.
1. Americans were used to the land
2. British had a long journey to send troops to America, that could take months at a time.
3. We used a tactic of Gurrila Warfare... The art of Hit and Run.
4. They tried to rule in a land that hated them.
Heres how this connects to Skyrim, and the Forsworn.
The Forsworn have always lived in The Reach, so they know the land well.
Forsworn live in the hills, Empire had 100's of miles they had to send troops.
Forsworn can hit a caravan, rather its a military or a civilian, it affects the Nords that own the reach. This is a large form of Gurrila Warfare.
The Reach is full of Forsworn, all of which hate anyone thats not them. Weary to the lone traveler that wanders The Reach.