Yeah, mostly because the other nations wouldn't judge the conquering nation. They didn't gang up on you to tell you what you were doing was immoral and wrong, and that if you didn't stop, nobody would talk to you anymore.
If you can defeat a country, it's fair game. And why does one conquer another nation? For the increase in territory and security (even if there's nothing in Ferelden they want - being constantly at war with Navarra the Orlesians would have an interest in silencing the old foe on the other side) for the increase of population (a non-issue if Blighted), securing trade routs (Orlesian ships must pass Ferelden on their way to Antiva), for resources (mining can be done, even if the area is Blighted, as said above), to generate a constant market for the products of your country, or to gain something of value in said country (The only entrance to Orzammar is on Ferelden soil - if the Orlesians want to trade with the dwarven kingdom directly, they must go through Ferelden - which they can't)...

Just pick one (sigh)...