And this is because of two of 4E's stated design goals.
1) that the player characters are heroes battling against the darkness.
2) that the implied setting takes place in a traditional fantasy world.
Yes all these renegades in a POL setting will be traditional fantasy types like humans, elves dwarves etc. The heroes are protectors of these sorts. Asking for anything different is going to lead to frustration since WoTC are sticking to these goals. Sure, individual campaign settings may shake things up a bit, but ultimately you're going to get heroes battling monsters in published materials.
All of this is moot, really, considering that there are plenty of published stats for humans, elves, dwarves etc. if you want to run an evil campaign that battles these types of creatures there's really nothing stopping you.
I see several disadvantages with this approach.
1. D&D Is, and always was a toolbox, especially now as the default setting is very vague. A PCs vs. everyone approach might fit a PoL setting, but there are many different types of setting in which this wouldn't be appropriate.
2. It adds to the "4E = combat" impression. With its heavy empathize of tactical combat 4E creates this impression among many people, even those who like this system very much. This is especially "problematic" as 4Es stated goal is to attract new players, and what impression would they get of PnP RPGs when the world resembles a giant arena where its the PCs vs. everyone else not in a city wall?
3. If WotC, instead of making good monsters unaligned simply does not publish good monsters it makes it harder to do some scenarios. For example what if the PCs want to team up with an angelic host or other good "monsters"? The DM has to create the stats from the scratch. (Also that monsters are not really able to fight other monsters is also a problem. But that is a different issue). That would only be a problem in a few games, but as I said in 1, D&D is a toolbox and imo should support as many different styles as possible.
4. It breaks the D&D lore. For example metallic dragons have always been good. Sure, 4E is about slaying sacred cows, but that cow didn't needed to be killed. If someone wanted the PCs to fight against metallic dragons he would have simply have such a dragon attack the party.
That metallic dragons are unaligned isn't the real issue. That is easily houseruled. The problem is that this is just a symptom of the real problem which is the design decision of not having (many) good monsters as there is no need for them. Imo there is.
I simply see no advantage in making traditional good monsters unaligned, but several disadvantages. No huge, gamebraking disadvantages, but (small, annoying) disadvantages nonetheless.