A post at The Incidental Economist considers the issue:
Workplace wellness programs discriminate. That’s what they do. Employees who adhere to a wellness program pay less for their coverage; those who don’t pay more. Wellness programs thus clash with federal rules that generally require employers to treat their employees even-handedly, regardless of health status.
The ACA, however, does explicitly make an exception for wellness programs. Nevertheless, the question of whether employers should be in the business of employee health is an important one do consider.