Absolutely. Its becoming more and more common. If anything, you have to prove eligibility during enrollment and continued proof of eligibility for dependents other than the spouse on a recurring basis.
Insuring ineligible dependents is FRAUD. That's STEALING from the company. Why would you fault a company for keeping their losses due to fraud to a minimum?
On the flip side, there are some situations that will come down in a very expensive fashion on non-custodial parents. When the court requires the non-primary custodial parent to maintain health insurance on a child (as a result of divorce/paternity proceedings), your insurance company may not consider a non-custodial child your dependent....
The court order is the parent's problem, not the parent's insurance company's problem..