It's like this, I'm British. While I don't live in G.B. I still think of myself as being British. The thing is The Republic is American and the sith have British sounding voices. The Sith are the bad guys in this game. Even though I play them.
Should I therefore be affronted that the makers of this game have used British sounding voices for the sith?
In the movies many of the Empire's officials had British accents. That's really the only reason the Empire has this kind of accent in the games. Also, whether the Empire (not the Sith, but the EMPIRE) is good or bad actually is pretty subjective.
If you want to look at an accent and feel affronted because it's used to represent a group of people that you may not particularly like, then World of Warcraft must think Jamaicans are nothing but a bunch of voodoo worshipping lazy layabout trolls. Catch my drift?
The Empire has an accent. It could just as easily have been German or Chinese or anything else.