Tae Bo workout caused a 39-story skyscraper to shake?

A couple weeks ago, people in the TechnoMart building in Gangnam, Seoul felt the building shake up and down. Following the immediate evacuation, the whole country's attention was on this building - what happened? What caused the building to shake vertically?

Well it turns out, at least this is what the scientists have determined, this is what happened. (CNN article)

Seventeen people performing a vigorous Tae Bo workout caused tremors that forced the evacuation of a South Korean skyscraper earlier this month, the building's owners say.
Prime Group, owner of the 39-story TechnoMart commercial-residential high-rise in Seoul, said 17 middle-aged people were working out to the pop song "The Power" by Snap on July 5 when their movements set the upper floors of the tower shaking for 10 minutes.

 Alright, a couple things, besides the obvious that they should build stronger buildings that can withstand physical shock and whatnot. (Earthquakes rarely happen in the Korean peninsula. When they do, they're always really minor... which is why most buildings in Korea have more lenient building standards than say, California.)

Why is there a gym, especially for a sports that mostly involves a lot of people jumping up and down at the same time, on a 39-story skyscraper? Shouldn't something like this be like, I dunno, on the ground?

And is it just me, or do you think they would've figured this out in like 3 days if they just asked Mythbusters to figure this out on TV?