Its actually not that surprising at all in my opinion ...
My local gym charges $60 for their personal trainers who have only completed a (very) basic personal training course offered by the franchise that the gym belongs to; I know a lot more about fitness and nutrition than many/most of the personal trainers at the gym (and that's not saying much). An experienced personal trainer with a physical education or kinesiology degree can set you back $100 to $200 per hour depending on their reputation.
Now it may sound crazy that people would pay that much to just use the Wii, but with people's heart rates hiting 140 bpm I highly doubt they are "Just" playing Wii Sports ... More likely it is Wii Sports or Wii Fit being used between other excercises. I could be wrong, but I suspect most people who are paying for this service would do so they could learn how to work out much better at home using the Wii (and Wii Fit) that they already own.
Just so that you have something to try if you own Wii Fit and want a better work-out ... Get an aerobics step from your local Walmart (they are not that expensive) and between strength excercises I want you to run in place, do jumping jacks, do step up and down on the stairs (forward or sideways), do burpies, or any other aerobic excercise you can think of for 90 seconds.
