Good point, however I think in this case it is about the bricks and not the house; when you use "free bricks" you can make your own bricks better or different than the original but you have to share your improved brick receipe with other people.

The house you build with the bricks however, that is yours to use as you feel fit; you do not need to share the blueprints with anyone if you so decide. (though I of course would like you to!)