Why should they give free keys to everyone?

Because it would be right right thing to do?

And it really isn't a paid product. On Steam when you buy the original game, you automatically get the VR version, and if you buy the VR version you automatically get the original version.

So they are literally not giving an update to the game to their GoG customers.

It is also clear why they are making it a separate game. On Steam there is this stigma with VR games. If a game has the VR tag on it, many people block it cause they don't own VR and they don't want to see it, or they don't want to even look at VR games that can also be played in non VR because they feel that the game was made for VR and ported to non VR later which tends to not be very fun experiences. So some developers are getting smart and they are putting them separately so that the non VR version does not have the VR tag on it.