I don't know why but I just love these "flash mob" instant musical numbers. I once saw one very much like this — same song, in fact — pop up in Vegas in the spot just between Harrah's and what was then the Imperial Palace. I have no idea who organized it or why. It didn't seem to be selling anything. The film Mamma Mia! was not playing at the time nor was there any ABBA-based show then in town.
But it sure made people happy. If I ran a big casino, I think I'd hire a troupe to wander around the place and just do these in unpredictable spots at unpredictable times…