Imagine admitting an unidentified object flying over your soil and your defence system not being able to force it down, what consequences that would have for your military, intelligence agencies and your government in general should the object belong to a foreign military. Unless someone has actually seen an alien fly one of these things, it's safe to assume that the unidentified objects are likely to be part of covert operations of either one of the competing powers or the United States military itself. Either that or the aliens really do perceive the entire world to be United States.