That's true. But at LGA, the CBP presence exists (at Terminal A) primarily for the purpose of clearing general aviation/charter flights, as LGA does not receive scheduled commercial flights from international airports which lack a pre-clearance facility. Of course, none of this precludes CBP officers from meeting a pre-cleared commercial flight at one of the other terminals, but that would be very unusual. Would be interesting to learn if that's ever happened at LGA.