I re-skimmed this thread and I don't think this point was posted:
We applaud/cheer at movies and at DCI Cinecasts probably for the same reason many of us post "shout outs" to our favorite Corps on DCP show threads. I guess anyone actually at the venue and posting on DCP could relay that "CrownBariDad said 'GO CROWN!'", but probably not gonna happen. I just think it helps those of us following the show (or watching it in a theater) somehow connect with the event when we can't be there. We also connect with those on the thread (or in the theater), too.
And maybe...just maybe...the Drum Corps gods will smile upon our efforts and help our chosen Corps in their quest for success. Again--probably not, but who knows?