amok: i guess it is beacuse the machines there only have the GOTY edition installed, so it includes all the expansions as well. So to have partiality (basically not favoring those who only got the base game) the user need to have bought the expansions to use the service. and I am not sure if there is a way to check this? So the easiest option would be to just go for the only version they know have them, which is the GOTY edition.
Could they have made two different version (base and complete)? Maybe?
Your probably correct. Meaning they would need a version for EACH DLC to cover all the bases... otherwise, say you own the base game and only one DLC? Well now you have to have a copy hosted for that too.
Easier to just make needing all the DLC a requirement...