Well one reason is because it is free. I believe if you pay for access to the internet, why should you have to pay to access sites that SHOULD be free. After all we spend a small fortune on our equipment. Surely that is enough to show your preference to one camera or another... Why should you forever have to shell out more $$$$ to talk to other people who have similar cameras to the one you use?
Secondly I do not see the value of paying for the other site. Honestly, what do you get for your money? If I am going to pay for something, I want my value out of it. Sorry, but I do not see the value of paying for a site, on the internet that I would not visit that often.
That is all I can think of for now apart from the fact that there are many communities out there that are helpful for photographers Pro or not, and are free as well.
So, you buy your equipment, that is not cheap. But to Nikon, is that not enough? Where does the grab for money end? I say it ends when they ask you to pay for a site on the internet
Don't get me wrong, I love my Nikon's. I also love my other cameras, but you got to draw the line somewhere