All themes (wordpress) that use WP functions are under GPL license, which means its free software ("Free software" is a matter of liberty, not price. To understand the concept, you should think of "free" as in "free speech," not as in "free beer.") Free software is a matter of the users' freedom to run, copy, distribute, study, change and improve the software.
Wordpress faq puts a good description on its license:
WordPress themes supplied contain elements that are derivative of WordPress's copyrighted code. These themes, being collections of distinct works (images, CSS files, PHP files), need not be GPL-licensed as a whole. Rather, the PHP files are subject to the requirements of the GPL while the images and CSS are not.
This is pushed very hard by Wordpress founding developer Matt Mullenweg. Even Thesis (after a long debate between Matt and Chris) is GPL, but its images and css is protected under their copyright. So technically, the theme code is free to use, sell, share, or edit, as long as your give the license and source.
The thing is alot of people who develop themes and plugins do not recognize this and will try to sue.
Most themes have their images and css files with the package, so using these things are in violation of copyright if you treat it as GPL, so as long as you dont use their images and css, technically its all legal.