Legally, there are no rules which state any geographical or political requirements which a state has to meet in order to join the Schengen area (there are such criteria for joining the EU, but this is a different matter). The only possibility to join the Schengen zone would be to enter into a treaty with the EU, which includes a clause that Schengen-related EU law would be applicable in the associated non-EU state. The associated state would only have the right to be consulted. Thus, the U.S. would have to accept each and every majority (!) decision taken in Brussels whether citizens of this or that country would need a visa for entry into the area or not. And the U.S. would have to accept that every third-country national holding a residence title of a Schengen state could enter the U.S. without even being checked. To BIL: Guadeloupe and French Guiana are not inside the Schengen area because this would e.g. require France to equip the complete rain forest border between French Guiana and Suriname, Brasil etc. with high-tech infrared sensors which would detect every monkey which is crossing the border - just in order to avoid ID checks on flights between there and Europe. I can understand why they do not invest any money into this.