Whether U.S. citizens enjoy U.S. Constitutional protections when abroad is an unsettled matter too. The recent drone killings of U.S. citizens fighting for ISIS abroad were done under the presumption that the answer is "no". They are not entitled to due process guaranteed by the 5th and 14th Amendments. If you extend that reasoning (not saying this is correct, just saying if you extend that reasoning), then U.S. citizens trying to re-enter the U.S. do not enjoy Constitutional protection until after they have been admitted.
That's why DHS trying to extend this territorial exclusion to a 100 mile bubble around U.S. entry points (borders and international airports) was so ridiculous and troubling. They were basically trying to make it so anyone within 100 miles of the U.S. border or an international airport did not have Constitutional protection.