The "Right-to-burn" Law describes a state law which covers the liability of prescribed burners given they meet certain training standards specified by the state. Details of this law and the degree of protection offered by it vary by state.
Source: Hardy, Colin C., Ottmar, Roger D., Peterson, Janice L., Core, John E., and Seamon, Paula. 2001. Smoke Management Guide for Prescribed and Wildland Fire 2001 Edition. Produced by the National Wildfire coordinating Group Fire Use Working Team.