I think you'll find this is not true.

In particular, in the Nook FAQ:

Your nook can connect to B&N Fast & Free Wireless networks in the United States. These networks are cellular data networks that are widely available. In areas without B&N Fast & Free Wireless coverage, you can rely on Wi-Fi to access network services. … Your nook CANNOT connect to other cellular data networks in the United States. It cannot connect to any international cellular data networks, including those in Canada and Mexico. …

I'd say they have created their own private cellular enclave and are in control of what comes and goes.

Regardless of their technical filtering, you should read your license carefully. I don't have a copy, but phrases like there is no charge for you nook's wireless capabilities do not imply that you can implement your own features and use their cellular service free of charge.