I've never imagine someone would asking this question right on that YouTube videos. That would be funny.
It's all about entertaining the multimedia contents to make it more attractive. If they let the video without the helps of that supporters sounds, it will be a boring video with only the wind or the ball kicked sound, and the players screaming. They even cover the empty seats with interesting design and pictures.
That was my first instinct too, that it was a ploy to make the game more entertaining since there aren't a lot of audience, I understand this kind of thing for entertainment, we watch a lot of shows with laugh tracks and we don't seem to mind so I guess that there is nothing wrong with this video. I am imagining it already, with no cheering and only commentary, I think that viewership is going to go down.