Not a sin tax but a thin tax

I was given a petition to sign to protest a proposal to assess a tax (the sales tax rate) on gym membership. Now, if the justification for proposing an extra tax on sugared sodas is that it is a tax on something that is bad for the consumer, so it is like a "sin tax," what would be the rationale for taxing something that is good for you? My guess is that they just assume if you can afford the gym membership, you can afford the tax. But that type of reasoning could also lead to adding a sales tax on private school tuition. If you really couldn't afford it, you would not have opted for private school over free public school.