Hey,
Back in the day you could put characters from your hand into your territory during battle, and in fact you could play a banding card, put a hero from your hand into your territory, and then band to it. So in that system there was no difference between banding from "in play" or banding from "hand or in play." I imagine that when we tightened up the rules, eliminating the ability to put a character into play from hand during battle, we left the loose rules for banding and eventually defined the loose rules for banding as the default.
Tschow,
Tim "Sir Nobody" Maly