Odds of someone copying your game and trying to make money off of it is low. Someone interested in doing that probably won't even care if it is legal so it doesn't matter if you make the source available. For example the rpg maker community has had people upload games onto itch and other sites when the games are available for free elsewhere.
Some commercial games have been open source. Marvellous Inc released their source code. I don't know if it was done on day one or not.
In most cases I suspect it won't matter as people won't bother to build it themselves. I do distribute open source games on this site but also provide them for free on my website.