That's what's so screwy about this whole thing.
Up until the moment a person is physically standing on US soil, they are not breaking any laws, it is the act of BEING in the country without INS status that makes an alien an "illegal".
Yet, the moment Cuban rafters physically touch US soil, they are covered under the Cuban-American Act of 1966, and receive special status from the INS, so that they are not "illegal".
That's why the USCG intercepts them at sea.