Strikes me as rather clever, actually. I'd expect the rate of absences to converge down on some stable value from the (high) base rate, so you'd probably need to do some experimentation to figure out what the stable rate is. Subsidizing a low monthly payment with penalty fees might end up being a better business model in the long run, depending on what the numbers turn out to be. But paying penalty fees feels like a failure where continuing to pay a monthly fee doesn't, so I'd expect it to be effective at encouraging people to work out.

The downside is that I'd expect a number of people to be shamed into dropping their memberships altogether. Shouldn't hurt the business model as long as it's not everyone, but it does change the expected income math.