This is injected into posts by add pop up blocking software, I think this one is from Norton internet security.
The latest version of the forum does have code in it to try and strip this from posts.
The problem is that new pop up add blocking software comes out all the time and the software companies issue updates where the code they inject is different.
Each time this happens I have to send a few hours installing the software on my own machine and coming up with new code to remove thses code injections that this type of software injects into each page.