The key difference here is the existence of South Korea. Sure, if the US were to waltz in alone, they'd be treated as the invaders they are. But every North Korean has been fed a steady diet of "Korea is one!" since birth and they're all well aware that their kinsmen in the South are materially better off in every conceivable way -- so when said kinsmen lead the invasion and form the new government, they're not going to get violently rejected in the same way.

The fall of East Germany is instructive: while that imploded internally instead of being triggered from the outside, there was absolutely zero violent resistance to West Germany coming in and essentially taking over. North Korea isn't going to be same level of cakewalk, but it's closer to this end of the spectrum than Afghanistan or Iraq.