I don't think there's anything wrong with sponsored ads. If someone downloads a theme and it has sponsored ads, they can either decide to use another theme or remove the ads. It's not like they're being forced on anyone.
Someone above mentioned WordPress is free and people shouldn't profit from it. I don't agree with that. People are profiting from Premium themes and plugins. People are profiting from custom theme designs.
Another thing is that sponsored themes are a great way for a theme designer to make a little money on the side. Web design is an incredibly competitive industry, and many designers simply don't make enough to live on for quite a while. Offering sponsored themes is a way for these designers to help pay the bills for a while.
One last comment. Sponsored themes can absolutely be used for people trying to build links to porn sites or spam sites or other "bad neighborhoods", but should we completely overlook the good that can come from sponsored themes simply because some people abuse them? Ultimately, people are free to make their own choices...to use a sponsored theme or not, or to leave the link in the footer or remove it...should we start making decisions for these people? I think we should leave it up to them.