They already volunteered. Fit service members are not self selecting for phony sugar because they are already obese. In other words, they are not overweight already; they have no reason to use artificial sweeteners. Logistically, it would be more convenient to study the military and their spouses and families.
It would also be easier to test HFCS sweetened pop versus sucrose sweetened pop. I have no more reason to trust the makers of HFCS than the makers of artificial sweeteners. The link in comment# 20 is a review article. I'ts quite damning of HFCS. I used to think all 6 carbons sugars were functionally equivalent. Fructose promotes lipid production in hepatocytes.
If fructose is the enemy then you'd have to demonize sucrose as well. There are two formulations of HFCS being commercialized today. The formula used mostly in soft drinks is 55% fructose and 45% glucose. Sucrose, of course, is 50% fructose and 50% glucose. Does anyone really think a 5% variance is responsible for all the alleged maladies?
The other commercialized formula of HFCS is just 42% fructose and 58% glucose. This formula is commonly used in baked goods, cereals, and other processed foods. If fructose is really the enemy, then the anti-HFCS crowd should be touting the benefits of this particular formula of HFCS over sucrose.