The determinations for US emigration control purposes are made by the US government, relying largely upon the information collected and transmitted, under law, by the airlines to the government. The airlines have no highly reliable way to know what the US lawful presence status is of even an ordinary foreigner, admitted or not by the USG at a US POE, independent of the USG doing the checks. De facto and de jure, the emigration control checks have been done by the government (not the airline); the checks are mostly electronic, but sometimes physically airside (e.g. at the gate) or even arguably sometimes physically done at the approach to the security screening checkpoint.