I completely understand why theme designers have sponsors for their themes. It is a lot of hard work to create a good web design template, especially if you have the added complexity of a content management system to account for. There needs to be some incentive for providing a free product. Not many people have all of their needs taken care of and are good-hearted enough to spend their free time making software that is available for anyone to use at no cost.
Because nearly everything is free to use on the internet and software is easy to duplicate, people have come to expect that computer stuff should be free. Many people who would never shoplift have no problem pirating software.
It is just too easy. But if somebody broke into their home and stole their computer with all the pirated software on it, I bet they would be outraged and vengeful towards the thief. That is just plain hypocritical. With all of the good open source software available, there is really no excuse for pirating software anymore.
Anyways, back to themes. Sponsors are a good thing. They provide incentive for theme designers to create high quality themes and then distribute them for free. If you don't want sponsored links on your website, there is always the option to pay a professional web designer to build a layout. While I believe it is perfectly okay to have theme sponsors, there are times when this crosses the line. One of these times is when a lot of hidden code is included in a theme for SEO purposes. Because this code invisible on the live website, most beginning users who do not look into the code will not know that it is there. While this practice doesn't always harm the user, it can bring down a user's pagerank and besides it is just sneaky.
Where things start to cross the line even further is in the choice of sponsors. There was a theme added to ThemeBot with a sponsored link leading to a penis enlargement site. But even penis enlargement companies need to make money too, right?...
Well, if there is one thing ThemeBot hates it is spam. If ThemeBot wanted a bigger penis he would have no problem finding pumps, pills and whatever else is out there. It is utterly idiotic to send hundreds of emails a day about this kind of crap. While the theme mentioned does have that stupid link in it, it is also licensed GPL. So, if you like the theme, you can delete the links completely. With any other license, this theme would not have been accepted.
I guess the bigger picture of this whole discussion is that there are some drawbacks to having everything for free - specifically, lack of quality and having to put up with spam. WordPress is considering disallowing sponsored themes in the WordPress theme viewer and many users have discussed this here.