What if the non custodial parent got Obamacare for a child they have no right to claim on taxes?

When my husband and I filed our taxes for 2017 we claimed my son. We later found out my son's father (non custodial parent) got Obamacare on my son. My husband and I had no idea he had done this. Well we are now being held responsible for paying for this insurance. Why is it our responsibility to pay when we did not do this?