Yes they could is what I think. That means they don't have Ad&d label on them and also if You don't use a lot of Ad&d mechanics and spells. I am not sure but from what I have seen in the first game it is not that Ad&d at all and more like a simple adventure type of game.
For money yeah it wouldn't be fair but there is always donate button, so for a game that is free you can distribute it for free with option for people to donate for its development.
Now for something better there is usualy that pay element, for example if You need some better graphics that are not free (like 3d models ect for example something like this https://www.assetstore.unity3d.com/en/#!/content/7065
) or anything else.
What I think is that it would be fair for some minimal price to cover the production cost. That means a possibility for designer to be able to ask for his players to cover some of his production cost. Any extra he could donate to developing team or anything else he choose.
I agree that free is easier but pay is making people motivated to work. Now as You can see in the new RPGs like NWN online or DD Eberron online You also can make most of the game free with additional quests or powers being P2P.
It would be like selling of books, after all authors have to make something of their work if possible is what I think.