Just wondering why the add-ons in F3 and FNV are follower-proof. Hard to code followers in the DLC or someting?

It's a design choice.

Is it really difficult to allow followers into DLC? I'm not saying to give them special dialogue or interactions, but just to allow them into the DLC map area.

Are you on Xbox? I'd say so by your signature. In that case you can't use mods which would allow exactly that. Anyway, this would be said mod for FO3:

http://www.fallout3nexus.com/downloads/file.php?id=15409

Yeah, Xbox here. But not talking about mods, but wondering about the development of it. Is it hard to do? Don't think so, if mods exist.

In programming it's not difficult, as has been said modders can allow companions into DLC areas. Its a design choice to ensure that the DLC is still challenging, by removing most of the advantages you're used to, it's the same reason a few take away you equipment or limit what you can take.