Hi,
When you purchase JoomSEF, its license (and Download ID) is valid for one year, so your license already expired on 1st July 2010. As version 3.7.2 was released on 9th August 2010, you (or your client) probably upgraded JoomSEF with expired download ID and the free version was installed, which caused the backlink to appear.
Your download ID seems to be still valid for the documentation PDF file, which is a mistake from our side and I'm sorry for such confusion.
Anyway, if you want to upgrade to the latest paid version (without the backlink), I must ask you to purchase a new one year license. After the license expires, you are allowed to use any version of JoomSEF you have currently installed, but you lose access to paid version upgrades.