Actually, I think I may remember why the direct outs may be different;
If you do a mix that includes panning in a stereo field your pan law changes the pan behavior. So you set the pan law and you do the mix. Typically we do mixes that aren’t the full stereo mix by summing various stems. These stems will often be stereo channels. If we allowed the direct outs to operate the same as the first direct out - the main out - then changing the pan law would affect not just stems and other mixes (which is why I would want to have it affect the direct outputs - because if a spec sheet says mono versions of stems/mixes need to measure the same as the stereo then that’s one way of combining signals that is different from if they accept a level increase due to the mono summing), but the change would also affect the mix feeding the stems.
I think I’m remembering it and thinking about it the right way and I think it might make sense.