Some out of network providers send bills to patients but don't collect. Some even tell the patients not to worry about it, although I believe this may be illegal.
In theory, the high reimbursements are so out of network doctors can get paid market rates. There's also a fair amount of propaganda from insurance companies on this issue which makes the reality hard to discern. Yet they pay 10-20% on average of the bills they receive, so it's just another way to punish the patient while both the insurance company and doctor are taken care of.