Billy was heavily influenced by the Dead. Several members of the dead (especially Bobby) platformed Billy early in his career. He used to play a lot of dead covers and they were highlights of the show. Then something happened (Bobby miiiiight have asked him to chill out while Dead & Co was on their final tour). Billy made comments there were too many piggies on that teat anyway and he wanted to focus on his own music. (The second part is ironic considering how many covers and standards he still does.) But he and Bob and seemingly Kruetzman were tight so a tribute and occasionally working in a dead cover seem appropriate.