NSF was started by Chris Bergen, a meteorologist by trade and a space exploration enthusiast, in early 2000s as a hobby forum (good old phBB) for people to chat about space and rocketry. I'm sure he couldn't even dream about becoming so popular so he didn't spend too much time coming up with a name (ie to protect himself against copyright infringement lawsuits). In fact I'm sure he would love to change the name now as they try to cover space programs all over the world (it's too late as people know them as NSF).

Trademark, not copyright.

The things real NASA produces are in the public domain and can be used for all purposes by anyone on earth, royalty-free, no copyright whatsoever.