Theories (as of 3:45 pm on June 6th):
1. Joey and John are legally required to be booked on every multi-act bill in Tucson, even if the headliner is the band behind "We Built This City".
2. It's a different Calexico. Back when the Gavin Rossdale fronted band Bush was terrorizing ears in the 90's, they had to go by Bush X in Canada. Maybe this Calexico is from England, and will eventually change their name to The London Calexico.
3. They're big fans of Savor, the Santana tribute act and are honored to perform after them.
4. Starship heard the incredibly obscure 7" where Calexico cover "Nothing's Gonna Stop Us Now" and begged them to participate.
I welcome your explanations.