Legalities using WordPress to develop sites for clients…?

Does anyone know the legalities to using wordpress to setup sites for clients?

Also using the plugins? The themes are custom so that is not an issue.

Another thing, what about the plugins that remove the visual elements to brand wordpress. I have seen a few plugins that scale down the admin panels features, and also remove all mentions of wordpress.

So, to me that is developed for people trying to do what I am as well? But is this accepted and allowed?

Thanks Everyone!