My experience of rhinos is that they can be very docile unless provoked. I was on foot in the Masai Mara a couple of year ago and came face to face with two of these creatures, I took a number of photos from just a few feet distance in fact I was so close at one point that with my 200mm lens attached I could only get the head of one in shot. The Masai Guide did say that he would not have allowed me to get so close if there were calf present.
In Japan perhaps the idea is not so daft. I mean what do you do if during an earthquake some large and possibly very dangerous animals escape from the Zoo? This was an earthquake practice so why not make it enjoyable as well as possibly instructive for those taking part.
I'm quite sure that the same people who are rolling around with laughter at this would be the first to point the 'why on earth weren't the staff trained to deal with such a situation?' finger if a rhino got loose and injured or killed someone.
The sight of the four shuffling feet under the fake rhino did bring a smile to my lips, but rhinos are unpredictable animals, and one running amok in a crowded zoo would be anything but funny.
It can happen, in fact it did happen at the South Lakes Wild Animal Park in Cumbria in 1997. The rhino was shot dead, so great was the fear of it attacking people.
As Condom says, the Tokyo zoo is in an area notorious for earthquakes - the prospect of a large wild animal escaping as a result of earthquake damage might not be so far-fetched, and the Japanese are pretty nervy about such things for obvious reasons.
It got the zoo some free worldwide internet publicity at any rate.