Yea, but I think the 5th amendment protects the employer and his property. If he wishes to hire based on a person's weight, tobacco use, apparent medical condition, or any other reason, he should be perfectly within his right.
After all, there is no "right to a job" in this country. Yet.
Be careful as there are a number of other reasons that have already been legislated against. Just look at the statements on most employment applications and thinking about the protected classes vs. health care. Businesses that try to hire healthy need to be very careful lest they find themselves in a pot of discrimination suits.
The problem rests with the insurance companies, not the employers. An easy way for employers to get around this is to work with the insurance companies and offer benefits to those who a qualified by the insurance company. Those who don't meet the insurance company's "health standard" can still work for the employer, but are not eligible for the benefit or pay a higher rate. This way the employer stays out of the discrimination mess. Health insurance is a benefit, not a right.
A simple changing of the language used would do the trick. Instead of stating the company has health benefits. They would simply state Health benefits are available to those who qualify.
Corporations are a creation of, and exist at the sufferance of, government, and in exchange for various tax and operational boons from the government, they are subject to various restrictions and regulation.