I'm going to reverse the question in the OP. What is right with certain entire classifications of creature being immune to sneak attack? It doesn't add to versmilitude to assume that there are no vulnerable spots on most creatures. It doesn't make the game more interesting or more fun. It annoys a lot of people.
What is the justification? How does it improve the game? Because I can see several ways it harms it.
First of all, you have no idea how many people it annoys. You, and others like you, may be in the minority.
Next, let's look at SA from 3rd edition.
Sneak Attack: If a rogue can
catch an opponent when he is
unable to defend himself
effectively from her attack,
she can strike a vital spot for extra damage.
Undead, plants, constructs, and swarms have never been described as having vital spots.
Undead: Animated by spiritual or supernatural forces. Their organs, if they possessed and still posses them, do not function. Skeletons do not have tendons, organs or anything else. Zombies continue to function even if their bodies are rotting and ravaged. The only undead that has vital areas are vampires and those are only taken advantage of after you have brought it down.
Constructs are basically statues that are brought to life. There are no gears, unless it is a clockwork golem. It is basically an animated object.
Plants: They have no anatomy so there is no vital spot. Severing a limb or two doesn't kill most plants.
Swarms: These are usually too small to hit and contains multiple small creatures that comprise one creature.
SA immunity, to me and others I game with, a challenge, consistency, fun, believability (not to be mistaken for realism), and danger. The way SA was described and the way these types of monsters are described, it makes sense that SA wouldn't work.