So here is my story...
I got a bill from a small clinic we visited a few times. I called to let them know that they were charging me too much. I would be happy to pay the addition fee over my co-pay, but not the total amount they bill the insurance for that was denied. 
She tells me, "Well that was the cash price, the amount we billed insurance was required under our contract with them."
"Wait, you are billing my insurance twice what I could just pay out of pocket, when we all knew the insurance company would not pay it? Did you tell me there was a "cash price" that was only a little more than my deductible?"
"Well sir, you hand us an insurance car, it's not our job to tell you"
"Who else would know?"
It's not just insurance companies...it's also the 1) incompetent underpaid staff working the counter or 2) small clinics trying to make as much as possible.