Is he only allowed to do that during Beat it?? He probably just felt like it... there is a thing a called spontanity!
What does that have to do with anything? Ah I see... the sinister musicians trying to hijack the show! It's just a performance of a song... a one-off performance at that... I'm sure they didn't really rehearse it all that much, as it was only done for one show... so in the end it seems like there was some mix up on how/when to end the song... but whatever, it adds to it... I love that performance... especially that part at 4:00 ! The way she looks at him throughout the song, the way he breaks into a laugh ("Oh my life baby... oh my life girl")! I don't see annoyance... I see that they had a really good time!