Well people buy Apple Watches regardless of the studies going on, however, Apple and in regards to this study Stanford University realized that hey, people wear their watches all day, so why not create studies that take advantage of the technology in the Apple Watch to potentially HELP people with these terrible diseases!
Apple does not need to give people Apple Watches for being in the study, nor does it need to pay people. Are people really getting that selfish that if they already have an Apple Watch and participate in a study they expect to get paid? I mean come on, you are helping your fellow human out! Think about this. One day, the results of the studies that Apple has going on with these universities could help you if God forbid you get heart disease or Parkinson's. Be happy people have zero issue doing this stuff for free, because it could help save you or a family member from these terrible diseases.
One last thing. Most medical studies pay well because they are somewhat dangerous. All you have to do in Apple's studies are wear your watch just like you would even if you were not in a study. You literally do not have to change one thing in your wearing habits.
Terribly selfish world we live in!