This edition of the game has a lot of things that don't make sense, but make gameplay faster. Cleave could have been done in 3e style where you make a totally new attack roll and damage roll, but that slows down the game, which is something nobody wants. So 4e made a few logical sacrifices - the end result is 99% the same, and so much faster to play.
Automatic damage is one of those weird ones. Our group faced these tiny crab creatures - totally harmless minions with a weak attack, but if you started next to one of them, you took automatic damage from each of them. Yikes! Didn't matter if you wore platemail or if you were encased in 3" of concrete, those SOBs got through.
4e, more than any other game system I've seen, requires for the DM and the players to work together in coming up with cool explanations of what really happened (i.e. provide appropriate fluff) and what the hp loss really means.
For instance, the cleave might not be a physical hit - you could just as well explain it as the enemy becoming demoralized as you slash his friend's entrails on his face.
Come and Get It has similar problems, as it also draws near invisible rogue-type enemies and weak wizard-type enemies who would rather stay as far away from you as possible, and it is left as a mutual exercise for the DM and the players to come up with an explanation as to how this end result came to be. Did someone push the wizard? Did the fighter feign weakness so the rogue saw an opportunity to strike?
I don't really think it's a problem but it requires a different approach to the game.