I don't want to be irreverent, but I read this on gnewsense wikipedia: "Since those issues were resolved on January 13, 2009, 3D support is now available again in the 2.2 release."

If all issues are legal problems can be solved like this. then what's the difference between gnewsense and other distros?

I hope I'm understanding the issue you raised correctly, but the primary difference between gNewSense and other distros is philosophical and not strictly legal.

There are still two remaining issues with most other distros:

1) They still continue to ship non-free software, mostly in the form of binary blobs in the kernel Linux.

2) They do not discourage the use of other non-free software and often include it either by default or in the "main" repository.

gNS doesn't advocate the use of non-free software and has made it a policy not to assist people in using/installing it. Instead, they stand for the principle and would even remove important functionality to preserve freedom (as they did with GLX).

if users intentionaly install nonfree software on gNS, gNS can detect and prevent this?

It is possible to install free or non-free software on gNewSense. But, gNewSense never recommends non-free software. Also, I don't recommend non-free software. :)

There is a lot of non-free software in many other GNU/Linux distributions. Even though the source code is available for a program, it might be non-free because of the license. I don't know why someone would make software but use a non-free license.

As for the software for 3D graphics, the program and the source code was available, but the license was non-free. The FSF asked the people who own the software for 3D graphics to change the license. And they did!

I wish more people and companies would choose to use a free software license.

Yes, gNewSense does not prevent the installation of non-free software but will not recommend it, link to it or provide assistance for installing it in it's official documentation.