The only reason that makes sense is that by hardsubbing, you are creating a derivative product, which is slightly more defensible to distribute, in the fansubbing gray area. Distributing the raw is much more obviously simple copyright infringement. Plus, distributing subtitle files makes it easier for bootleggers to include them on pirate DVDs.

And >>5, I think >>4's point is that subbing groups have no rights to use the raw files, and thus it is pretty hypocritical to complain if somebody uses their translations without permission.