Mixture,
Perhaps you are overlooking the fact that the vast majority of corporates access the internet via a proxy such as websense that takes on most of that functionality - unlike home users, who have to provide it themselves.
Also, it is not by any means unknown for IT departments to deploy a few browser plug-ins for specific reasons - e.g. ad blocking. It's not a big deal to add these via a software / application deployment tool to a standard build / image.
SD