Depending on how the plan is worded, they MAY be obligated. You can't say categorically that they are not. And you have not yet shown me a law in any state that PROHIBITS a company from allowing their employees to cover divorced spouses.
I never said it was prohibited. (However, at least in NJ it IS prohibited).
Health insurance is an employee benefit which costs the employer money. They do it for goodwill for their employees. There is no reason to insure an ex-spouse unless required by law.
I've worked for a number of companies and NONE of them would offer insurance for ex-spouses.
However, if you insist:
New Jersey
http://www.divorcesource.com/NJ/ARTICLES/sliwinski55.html
"A divorce causes major issues with health insurance benefits. Many families have an employer provided and/or paid for health insurance benefits that cover health insurance for the entire family. After a divorce, the spouse with the family health insurance coverage can no longer cover the other dependent spouse. They are no longer "family" members who can take advantage of the employer-based health insurance policy. There is no way around this unfortunate reality."
Or, let's look at FEHB (for Federal employees):
http://www.opm.gov/insure/health/faq/divorce.asp#2
"Your spouse is eligible for coverage while you are in the process of getting divorced and even while you are legally separated. Your spouse loses eligibility for coverage as a family member when your divorce is final." (the spouse MAY be eligible for his own coverage as a former spouse of a federal employee, but the employee can't have him on her policy)
General rule:
http://www.wife.org/lowenstein.htm
"If your health insurance is through your spouse’s employer, once the divorce is final you will need to obtain health insurance for yourself."
Another general rule:
http://www.millenniumdivorce.com/articles-divorce/art29.asp
"Health insurance providers rarely allow a party to continue to provide insurance coverage for a former spouse following a divorce."
More general rules:
http://www.buzzle.com/editorials/11-20-2005-81868.asp
"After a divorce, the spouse with the family health insurance coverage can no longer cover the other parent. They are no longer "family" members who can take advantage of one health insurance policy."
Or:
http://www.18884mydivorce.com/pub/Alimony/health-insurance.htm
" the couple can agree to keep the non-primary spouse on the plan after divorce OR the court can order the coverage to continue. The actual truth is this type of arrangement is impossible. After divorce, the former spouse is an independent adult and is not eligible to remain on the same health insurance policy. Even if the two of you agree to this type of arrange, the insurance company will put a stop to it and drop the non-primary person from the policy"
Rhode Island
http://www.articlesbase.com/advertising-articles/health-insurance-coverage-for-spouse-after-divorce-in-rhode-island-1887804.html
At one time, it was allowed, but this rule has been watered down by the courts since it is pre-empted by ERISA.
I think the problem is that many states talk about continuation of health insurance, but they're talking about COBRA. IL is representative:
http://insurance.illinois.gov/HealthInsurance/continueSpouse.asp
The law requires that continuation be offered in the event of divorce. But once you read it, it is clear that they're talking about a COBRA-like policy. The spouse does NOT stay on the employee's policy, but can rather be given their own policy for a period of 3 years after the divorce. This is essentially the same as COBRA and is NOT what you are suggesting (that the spouse stays on the employee's policy).
So far, I didn't find a single state that even allowed the spouse to stay on the employee's policy, much less required it. Every state that I reviewed was like IL - they offered the spouse to get their own coverage under a COBRA-like scheme.