Yeah, because what you want is a insurance corporation that tracks your health and adjusts your premiums directly on your stats.
Sure, 25% up front discount, but when they find out your health isn't great then boom, suddenly you are paying 50%, 100%, 500% more the originally? The the problem is they decide what good or bad health is, you can be fit but have a minor heart murmur your watch will pick up, suddenly you are paying 250% more the next time your insurance is renewed even if your own doctor says its nothing to worry about.
And you might think, "well I will just leave Aetna and go to another insurance company", only thing is they are all in cahoots so once they know you are at risk with poor health your records are shared with all insurance companies and suddenly you can't afford health insurance anymore anywhere.
And guess what, your employer will have access to your health records and at-risk employees might suddenly be laid off for whatever reason because a company doesn't want people that might drop dead or be a burden of long term disability leave in the future.
It amazes me how much people will jump at a marketing promotion to get something free or get a perceived savings on cost without thinking one damn minute about the ulterior motives. NO corporation wants to earn less money from you, there is ALWAYS an ulterior profit mongering scheme with anything any corporation does.
Aetna can spin this all they want, the reality is that they will use this information to increase profit off of at-risk customers OR simply cancel at-risk customers that prove to have health stats that might indicate future liability to the company when a customer might actually need to use their insurance for health reasons.
In the end, this is yet another greedy organization that is looking to make more profit by violating your privacy and it is surprising that Apple would allow this considering how vocal Apple has been to not collect consumer data for profiteering. But right now Apple just wants to sell more Apple Watches and could care less about your privacy by allowing apps such as this to tie directly into another corporation's profit scheme. Dangle a shiney bauble in front of you, suddenly people want to share their private information with a corporation.