Simply put, that isn't their place.
Defending yourself in court is expensive. If the NRA tried to become the knight in shining armour on these lawsuit, the expense would be huge. Contrary to what most people think, the NRA doesn't have all that much money...not compared to a John Edwards, for example.
Also, if the NRA tried this tactic, then the anti-gunners would flood the courts with lawsuits with one aim, the drain the NRA dry.
The NRA does one thing well. It rallies the troops and uses them to put pressure on policians. Politicians react to pressure, and no pressure is so great as the threat of being tossed out of office.
Everything takes money. People hate the NRA mailouts, but they generate more money than they cost and they keep the NRA base involved even if you only throw the stuff away. It does put the NRA in your head.
If you don't belong to the NRA, you (my opinion) aren't really serious about preserving your gun rights. I know there are some who have some beef about the NRA, but who is better, on a national scale, at protecting your rights.
Also, if you don't belong to your state rifle/pistol association, same thing, just on a local level.
We can all just bitch about the politicians or we can also do something about them.