Ah, you mean killing the sickly? (joke)
I think in order for anyone in a position to consider adding gym membership as a cost reducing means towards reducing medical costs, there would have to be multiple substantial (expensive) studies done. There are a lot of factors to consider, including injuries that occur at the gym, how much going to the gym X amount of times for Y amount of time doing Z type of exercise would decrease the risk of T number of common and expensive to treat diseases. You'd need to select groups of thousands of people from all each age group in the US for each variable change, and you'd need a sizable control group. You'd also need to observe them for... maybe 50 years?
I see where you're coming from here, though I do think it'd be better to support education for people into healthy eating and the importance of working out rather then subsidizing businesses into making more and more money. Education has a much better payoff for us rather then shareholders just wanting their company to lobby for government subsidizes.
I don't know what incentive you'd get out of abusing the system? If someone is handing out free gym memberships to all of their members and you grab one just to skew their data I suppose? There wouldn't be any direct financial benefit. I suppose you'd make your insurance company pay an extra $10 per month, maybe less as I'm sure the insurance companies could A. open their own gym facilities or B. negotiate some crazy low rates for signing up a million members.
I don't mean an incentive from people, I mean more of a benefit towards gyms just offering people food in order to just show up to the gym once a month (or something like this) So people aren't actually working out, the gym is just reciving money for people to get more food or something along these lines.
You can't get people into a gym by making it cheaper -- places like Planet Fitness (in the US) show that even if a gym is cheap people will just sign up and not go.