Johnny Cash was an early fan of Bob Dylan, but Johnny's cause was the plight of Native Americans. He did an album dedicated to the subject (Bitter Tears: Ballads Of The American Indian).
Merle Haggard is most well known by the general population for his "Okie From Muskogee". "We don't smoke marijuana in Muskogee"; I guess his good friend Willie Nelson didn't live there. ;-)
I knew a guy who did sound for The Circle Star Theater in San Carlos, California (a half hour drive south of San Francisco). A bassist I knew went to the theater to see Merle sometime in the 70's, and our soundman friend asked him if he wanted to meet Merle (well duh ;-). They went onto the band bus, and the bassist told me Merle and his band were sitting around the kitchen table, a huge mountain of blow being shared by all. I guess Merle knew America was itching for a redneck anthem, and didn't mind a little cynical cashing-in on that market.