They don't. It was a sports team charter. I forget who it was, but I believe it was for the Oakland A's, who played the Mariners that weekend. I saw it on the schedule that day and thought about going to photograph it, but was out hiking elsewhere. It's not uncommon to see charters like this at BFI
Pardon the self plug:
I've also seen charters there from B6
, and AC
, plus some strictly charter carriers like Miami Air.