The radio host Casey Kasem, best known for hosting the American Top 40 countdown and for being the voice of Shaggy on the cartoon show “Scooby-Doo,” has died at the age of 82. Beyond his radio work, Kasem was an Arab-American activist who championed a number of progressive causes, from veganism to fighting media stereotyping of Arabs. In the 1980s, Kasem was arrested at an anti-nuclear protest in Nevada and slept outside in Los Angeles to draw attention to homelessness. Kasem had suffered from Lewy body disease, a form of dementia. He died Sunday at a hospital in Washington.