Co-pay for none selected hospital stay

What is the name of your state?CA

I have Pacificare HMO for my family health insurence. It is through my employer. We had a new born this June and my wife stayed in the hospital for two days for the maternity. Three month later, we got a bill from the hospital asking for $500 co-pay. Since this is not a selected hospital by my insurence. This part of the cost is not been picked up by the insurence.
My question: We have no idea about the difference(selected or none selected) before and even the hospital had our insurence card at the time of servise, they did not mention anything about this issue. Shall we pick the tab? is there any law concern about this practise?

Any advise is greatly appreciated.

[enjay]

Steve, it's your responsibility to know the particulars of your HMO and where you should and should not obtain care. The hospital doesn't care if you will be subject to a deductable or not. You are responsible for the $500.

[cbg]

Nor is it possible for the hospital to tell from your insurance card whether your plan has a co-pay or not. The same insurance carrier can have many different plans, and the network is not the same for every plan.