DLC is not the same as an expansion pack. Most games seem to use features that are already finished when the game goes gold as DLC, holding them for ransom to make a few extra dollars without doing any further work. Dragon Age was a horrible example of this. Horse armour was a bit shameful.
SI is much more akin to an expansion pack, I am all for that, just like MW's Tribunal and Bloodmoon.
Many examples of DLC these days would have been released as free mods a decade ago.
I don't see the difference. DLC can be anything from horse armor to a full on expansion pack. It's all just Downloadable Content, which is a bit of a silly industry construct in and of itself. For example, Undead Nightmare for Red Dead Redemption gave you a whole new and rather lengthy Single Player campaign with new enemies, weapons and things to do on a pretty big scale. Along with new multiplayer features. It was a mere $10 and is generally thought to be one of the best DLC releases in years. It's more or less an expansion, but it's still classified as DLC. It's a misnomer and I don't really see a need to mention them seperately. Especially since the topic at hand is still applicable since, expansion or DLC, Bethesda still tends to make them exclusive to the 360 for a rather lengthy period of time.