The "reason" is the string which is used for the ad blocking logic. In this case, it is "adid=". This string was removed from the default ad blocking list before NIS 2003 / NPF 2003 were released - so you're either using an external test, public beta, or an internal copy.
If you go into options / Norton Internet Security options / Web content tab / Ad Blocking tab, you can see all the strings which are used to determine that something is likely an ad, and thus removed by the ad blocking. There are two things you could do:
1: Select Defaults in the left pane, then find "adid=" in the right pane. Delete it. This will prevent the blocking on all sites, but not block any potential ads with "adid=", which may be fine for you.
2: Click the "add site" button on the lower left, add the site you want (forums.anandtech.com, for example), then click the "Add" button in the ad blocking tab. Add "adid=" as an html string, and select it as a Permit. This basically overrides the default block behavior for the string for that one particular site.
I'll refer this to the tech support organization to make sure they have documentation on how to do this in the knowledge base. I would expect that they do, but can't be sure.
Brendon Woirhaye
Sr. SQA Engineering Manager
Symantec Corporation