I will attempt to answer your question directly, rather than going off on a tangent or creating some nuanced, "deeper insights," response - since by my reading of the responses so far, I see people writing very emotionally toward what they perceive are the ills embedded in the system. Everyone already knows that healthcare is screwed up in the US, right?
Take the case of an employer with 50,000+ employees. Why 50,000+? Because as of mid-2015 this is the only set of employers where the economics have shown that wellness programs create an ROI. Below that, accountants have not been able to demonstrate a clear economic investment across the board, it is much more on a case-by-case basis. So if you are below 50,000 employees, the answer to your question would be, "In general, there is nothing in it economically for employers, other than they feel good, their HR department is being hornswoggled by salespeople, or they have done some clear, good accounting with a record of wellness programs going back years which have demonstrated some positive correlation between better health and some indicator for the bottom line of their business."
If you are talking about the 50,000+ employee category, then in general there are economic benefits which can be proven in the world of accounting.
Health insurance companies (or more appropriately termed, Health Management Organizations, HMOs) quote out a given employer's rates based upon the risk pool made up of all of the employees. There is little to no leak of information between the HMO and an employer, because the information used is classified as Personal Health Information (PHI), which has very strict standards for how it is stored, the loss or misappropriation of which is a felony that can result in prison time. The employer does not see this PHI - that information is shared between healthcare providers (hospitals and clinics) and the HMOs. So when an employer goes to an HMO to get a quote, the HMO takes all of the PHI they have on the individuals within that employer's organization, and creates a quote based upon how they see that risk pool.
If an employer with a large number of employees, let's say Nationwide Red Dot Retail Store, Inc. (NRDS Inc), partakes in a given wellness program, they will receive a discount on that underwriting based upon the types of wellness programs they provide for their employees. These discount offerings are determined by data scientists and project managers who work at the HMO, and are geared toward maximizing profit for the HMO. For example, those data scientists may find that by offering people $20 to all employees to participate in a health survey, the number of smokers within that pool of employees goes down by 0.5%, which means that the cost to cover them goes down by 1%, which means they would offer some discount less than 1% to NRDS Inc. However, the accountants and data scientists at NRDS Inc., are no dummies, so they may take a look and see that their employee turnover rate is X%, so that particular offering doesn't make sense of them economically, because by next quarter they will have a whole new set of smokers they will have to pay money to, as a part of the wellness program, only to have them leave 3-4 months later.
Wellness programs are meant to use psychology to trick, not force, people into changing their health habits. People do not react well to being forced into things. Americans particularly, do not react well to being forced into things. You may read some other responses to this question giving you an impression that people are being penalized for their decisions and choices.
Here's a straw-man argument for you: let's say there was a country called, "Amazing Programmerlandia Island," and you could hire the most incredible, genius, friendly programmers who know tons of languages to help you with your startup there for $10/hour. There's just one problem. About 50% of them love to do Crocodil, and it is illegal to check whether they are addicts before you hire them, and illegal to fire them just because they do Crocodil, and you have to pay for rehabs. Would you take any opportunity you could to, "hack," the system and try to lower the number of potential Crocodil addicts in your employee pool with some back-door system? Of course you would, this is Amazing Programmerlandia we're talking about here, tons of money to be made for just a tiny investment!
This spurious example basically demonstrates the mindset of HR departments when they buy into wellness programs. The economics change based upon the size of the company, and the numbers a company may have - it is not an, "across the board thing."
To find out more about the ROI of Wellness Programs, I recommend you Google individual companies names, and reports on the ROI from that particular company, from that company's point of view, not from the HMO's point of view, or from an independent paper (from a place like RAND) or from the Government. I could see the Federal Government funding studies which support wellness programs regardless of their true ROI, and asking the HMOs what they think is like askign the fox to guard the chicken coop.
There is no real tax benefits that I am aware of for engaging in wellness programs, the potential returns are only based upon lowered underwriting costs - although if I am wrong on that, please correct me.