it probably is a cultural issue as well, whether to boo or to sit out a performance. having attended performances in quite a few parts of the world (including north america, china and europe), i generally would say it is a matter of cultural attitudes...someone mentioned in one of the posts how fascinating a strong "theatrical" response can be - loud, over the top booing is probably taken less seriously in a country like italy, by both audience and performer and seen more as an act of participation in the artistic discourse that is being established. In germany wild shouting at the end of a performance ís likely to be regarded as offensive behaviour.
Personally i enjoy the italian way. i think if it is understood on both sides - performer and spectator - that this is a dynamic, enjoyable ritual with a spark of good humour all around, then booing is acceptable...and i would say it adds a lot of passion to the discussion, which is only to be encouraged.;)