Yeah, people were expecting him to perform. But there was never anything announced, that was just made up prior the WMA's and never confirmed. The day after the press bashed him for that. He couldn't do it right. There was Chris Brown performing a tribute which was announced, maybe they confused it but then it was too late. The press just needed another plug to butcher him, writing people boo-ed at him for not performing (joining the children choir for WATW was hardly a performance).
Having been there in person I can assure you there was noone in the crowd booing at him. In fact, the opposite: it seemed as if the whole crowd was just there because of HIM. The Earl's Court (the venue where the WMA's took place) was filled with 15,000+ people from all over the world (indicated by so many flags from Latin America, to European ones and so on) cheering for him throughout the evening, between each act they applauded and then screamed for MJ again. It was not a live-broadcasted event so there were always like 2-3-minute-gaps between each act or award presentation. During every gap people started chanting "M- I-C-H-A-E-L! M-I-C-H-A-E-L!". Anyone could start and the whole arena would respond (a friend of mine and me being one getting one started, lol). Even a Mexican wave hit the hall. Imagine there's an event lasting now 2 hours and no Michael in sight, still created that atmosphere. The later the evening the more the crowd expected him. But it took longer and longer, people got impatient. In between performers on stage ie. Nelly Furtado, Chris Brown, Rihanna, and the likes. Beyonce – who did btw. a pretty decent medley at the beginning (back in the day when I liked her music more than today) – entered the stage and everyone knew the long awaited moment was just seconds away. Beyonce presented Michael with the 'Artist of the Millenium' award, and finally Michael appeared. Electrifying atmosphere, the man simply walked into the spotlight yet the crowd went nuts. Just like back in the day. Obviously Michael needed to begin speaking again and again as the fans cheered so loudly.
The rest can be witnessed in the video posted by the OP or check the others on youtube.