I think this is weird. Lets say, Wordpress is a commercial product. You won't even think a minute, that you didn't have to pay for it, even if you just use one

I think this is weird. Lets say, Wordpress is a commercial product. You won't even think a minute, that you didn't have to pay for it, even if you just use one feature. The same goes for the GPL. And if I understand those "app theme" correctly, they use the backend of Wordpress. For storing data, for administration of the data, users, rights. So I think the FSF is right. But the GPL doesn't say, you can't sell your themes, it just says you can't deny other to do, what you did. That is all.