IMO they're separate this time this because:
1) Konami, like every other JP publisher is desperate for money and wants to milk the few things that sell as much as they can.
2) The full game is taking forever and won't be ready until 2015, so they need to at least release a demo to keep interest.
3) The full game is taking forever and won't be ready until 2015, so Konami needs SOME PROFITS in their 2014 financial year
And then there's Kojima's reason which is:
1) MGSV PP is a hugely vast open world game that will confuse players used to MGS series (especially JPN players who don't get many open world games), so GZ is a way to start with a smaller sized sandbox world and get to grasps on how to play MGS in a sandbox before being dropped into the giant world that is PP.
I'm guessing there's a grain of truth in what Kojima is saying, but he's probably exaggerating to give a reason to Konami's milking.
That being said, I don't really doubt GZ will be worth at least the $20 price. Kojima is good at making meaty, replayable high quality game experiences. If GZ is 2-4 hours replayable with depth and PP is a 15-25 hour big campaign, I don't really have a problem paying $30+$60. Most $60 games these days are like 6-12 hour campaigns.
