The problem isn't the insurance negotiating a discount - it is that if your insurance has agreed to pay 80% of the cost of your care, and then is able to negotiate a discount, you should pay 20% of the discounted price, not 20% of the pre discount price.
I've never not had my co-insurance portion be anything but the percentage of what their negotiated rate was. I don't know if it is fraud, but breach of contract would probably apply if you did (assuming it wasn't in the fine print).
