From the story: "In a statement Thursday, Mary Martini, district engineer for the Nevada Department of Transportation in Las Vegas, said that while the state maintains the fences along I-15 to designate the right of way and control access, it is always the responsibility and liability of the owners to control their animals."
So if the state did have a fence designating the right of way, and Bundy allowed his cow to ener into the fenced right of way, then he's liable.
Did you read pp #1 in my link?
Are you from an open range state or really familiar with open range laws?