First off Guantanamo and Abu Ghraib, various tracks were played repeatedly at prisoners including Matchbox 20. IMHO two ideas explain this;
1) The guards simply wanted to test out if listening to Barney drives you nuts for a bet.
2) The tapes were doctored with subliminal messages to encourage cooperation with interrogators.
Woodstock, two of the last bands on were Limp Bizkit and Red Hot Chilli Peppers who performed 'Break Stuff' and 'The Roof Is On Fire' respectively. However the real reason for the bad crowd behaviour may be that there was a high ticket price for the whole festival yet inside alot of the concessions were charging top dollar. The organisers had clearly assumed an older, more moneyed festival goer then booked acts that appealed to penniless kids. No excuses but a big mismatch...
Music in general does shape mood. But you can't cheer depressed people up just by playing Bubblegum hits at them. Someone who's down needs music that says 'you can do it, play the system and win' not just 'happy, happy, smile, smile'. Where todays metal bands get lost is showing how shitty the corporate world is without reminding people that they can make an alternative for themselves. The original hippy message of 'revolution in your own back yard, do it yourself style' needs finding again...
'The sound of the city seems to disappear'