Tsuyoshi's very serious when it comes to music... He's kind of "rebellious" in a way too... I think he might have been upset at the media too, so he just left the stage as a way, not only as to show how angry he was but, to prove a point...
But yeah, i agree that he should have thought about how "the show must go on"... Maybe he could have found a way to pull together another, even better, encore and at the same time, find a way to prove his point rather than abandoning the stage... and leaving Koichi confused and all...
it was a bit cruel and maybe a little bit selfish to do what he did, but i understand how he felt... after all, music is about expression...