Yeah, but he only GOT steel-hard skin as an adult - he didn't grow up with it, or even gradually acquire it, as Superman did. Therefore, his immediate instincts are still the same - he's not thinking 'Huh! Snakes! Stupid people, don't they know snakes can't bite me', he's thinking 'Sweet Christmas, SNAKES! BITEY, BITEY SNAKES! DON'T LET 'EM GET ME!' He snaps out of it within seconds, of course, but you can hardly blame the guy for having an 'AAIIEE!' moment first.