Cook butter beans according to package directions; drain and cool completely (about 20 minutes).
Meanwhile, preheat grill to 350° to 400° (medium-high) heat. Grill corn, covered with grill lid, 15 minutes or until done, turning every 4 to 5 minutes. (Some kernels will begin to char and pop.) At the same time, grill onion and bell pepper, covered with grill lid, 5 minutes on each side or until tender. Cool all vegetables completely (about 20 minutes).
Cut kernels from cobs. Discard cobs. Chop onion and bell pepper into 1/2-inch pieces.
Stir together mayonnaise and next 5 ingredients. Stir in tomatoes, corn kernels, and onion and pepper pieces. Add salt to taste. Cover and chill 2 to 8 hours before serving. Store in refrigerator up to 3 days.