好像確實是個灰色區域，我查到這個英文文獻：

http://www.bennettlawfirm.com/oon.htm

還有這裏：

http://community.lawyers.com/forums/p/102676/498444.aspx

Waiving Co-Pays

The most common ways physicians reduce the cost of care for patients are waiving the co-pay ("insurance only") and giving the patient a discount on the care. In most situations, both private insurers and the federal government ban waiving the co-pay. (Medicare has some provisions allowing the co-pay to be waived for documented indigency.) They do this because the co-pay is meant to discourage casual trips to the physician. The theory is that making the patient share the cost of treatment will make the patient a more sophisticated health care consumer.

To date, there have been no reported cases, prosecutions, or settlements solely based on professional courtesy to health care providers. Looking at general patient care, rather than just professional courtesy, there have been private insurance fraud actions based on illegally waiving co-pays and/or providing discounts that were not passed on to the insurer. There have been Federal actions for the same violations, as well as for using waivers and discounts to induce Medicare patients to use other health care services.

感覺是in-network的醫療機構會更小心一些，因為他們和保險公司之間有contract，不敢隨便waive co-pay或者discount，違約了可能會有lawsuit或者失去in-network的資格。但是out-of-network的好像就比較失去規範暗箱操作的很多。。。