I'm not sure what the big deal is, or why they think it has to be Sith Assassins to do this. It's not an exploit, but a quest.
Chapter 4 bonus to be specific. Kill a few creatures and get some credits.
Reset quest and repeat. It's no big deal.
And for the record, no, I have not partaken in this myself.
My theory as to why they are using Assassins is because they can quickly use Phantom Stride to get from mob to mob. However, the speed boost from this method is minimal compared to other classes.