There seems to be lately some sites that offer 'free' WP themes where some of the code has been ofuscated.
In some cases, this is to force you to advertise for them at your expenses. These are the less worrying of the situations and it can easily be fixed by either decoding the footer.php or erasing its content all at once.
Some other situations, though, are of much more concern. They implement some sort of validation by using a tracking pixel or by sending information from your server and, in return, they update the links that you show up. Technically speaking, this is called a trojan horse. An example of this would be freewordpressthemes4u dot com.
Some others create a file called contact.php that, together with some code in functions.php, is used to spam your users. Much in the way that a botnet would do. An example of this would be qualitywordpress dot com and wpthemesfree dot com.
I encourage all members to post here those sites that are offering themes with ofuscated code.
PS: When confronted with questions, both sites mentioned above declined to provide an answer after a long e-mail conversation posing as a genuine customer.