One salient fact in this operation--KDCA is located in Class B airspace and not just any Class B airspace, the DCA Flight Restricted Zone. No traffic except specially cleared airliners and few GA that require very specific controls, including armed guards (all this is public info), so the chances of any "pop-up" traffic or uncontrolled traffic are nearly ZERO! Any planes approaching the FRZ will be intercepted and followed 24/7. There would have been far greater mid-air or ground threat to the airliners had this event occurred at, say, Memphis or Minneapolis at night. So, the only conceivable threat to these airliners might be ground traffic, which is pretty unlikely as these two flights were the only operating. So, the safety implications are pretty small. Legally, the FAA might have a case about the pilots landing w/o a clearance, but considering the circumstances, that ain't happenin'
GF