Corporations are sometimes users, and individual people often benefit from denying access to the source code of software that is used to provide a service.
Providing a service is fundamentally different from providing software.
The AGPL doesn't require you to provide the source for software that you're using to provide a service - only the source for software where the software is the service. A bug in software I use is equally annoying when I'm using that software over a network connection as it is when I'm using it on my local machine.
To the detriment of the users of that service--which are the people that the FSF aims to protect.
My customers, for example, to whom I provide service, are not infringed upon in any way because I use private forks of GPL software in the provision of my services.
The issue is that I—and, more importantly, your customers—must simply take your word for it, since we're unable to confirm or deny it for ourselves if you don't make the source available.
When dealing with large corporations, taking such statements at face value is pretty foolish.
You might not want to admit to yourself that your business model is based on infringing on the freedoms of these users of GPL software, but that's exactly what it is.