Xyzzy is, of course, correct.

[MENTION=160992]Pen[/MENTION]dergrast, you are correct that a citizen of the United States cannot be denied entry into the United States. (And did you know that certain other persons who are not United States citizens also may not be denied entry into the United States?)

But the prohibition on denial of entry is not established by constitutional law or statutory law. Rather it is established case law, primarily Nguyen v INS.

However, that rule only applies to decisions made by immigration officials at an actual port of entry into the United States. It does not apply to decisions made by airlines, or airline employees or airline/airport security contractors located outside of the United States.

I can understand the point of view or impression that an airline denying boarding onto a plane in Europe (or some other continent/country) seems to have the same practical result as a government official denying entry into the country at a port of entry, but in fact these are two entirely different situations. Denied boarding onto a common carrier is NOT that same as being denied entry by an immigration official at the port of entry. Fikre v FBI is the case that establishes the government including a person on the no fly list (which thereby results in denial of boarding) is not a denial of rights and is not the same as the government denying entry to a citizen at a port of entry.