好像确实是个灰色区域，我查到这个英文文献：

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

还有这里：

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.

感觉是in-network的医疗机构会更小心一些，因为他们和保险公司之间有contract，不敢随便waive co-pay或者discount，违约了可能会有lawsuit或者失去in-network的资格。但是out-of-network的好像就比较失去规范暗箱操作的很多。。。