What if you land on a pointy surface ... gravel, sand or sharp stones? Sure it's good protection, but let's think this through first.
(I am convinced that regular bicycle helmets are crap, as there are no decent standards or crash tests being done with them. The least I would call an improvement is a downhill helmet, a motorcycle helmet would be preferred as speeds for severe crashes are quite similar.)
As you can see, it's fairly rigorous. The main criticism is that by setting strict rules it discourages innovation. But that doesn't mean there are no decent standards or crash tests.
No helmet can stop concussions. If they could, the NFL would redesign football helmets and save themselves a lot of trouble. G-forces are G-forces, whether you're in a helmet or not.
EDIT: There are a lot of very smart people working on this problem. But it's a hard problem.
From the Associated Press:
In a series of interviews with The Associated Press, representatives of the NFL, its players' union and the four equipment companies that make every helmet worn in the league all agreed there's no football helmet -- in production or on drawing boards -- that can eliminate concussions. And there might never be one.
to patdennis (apparently there is a max level of replies): I think you underestimate the amount of force an NFL player takes when they get hit. NFL helmets stop 99.9% of concussions. If a cyclist wore one they would not get a concussion falling off their bike at 20 km/h or even getting hit by a car. NFL players get concussions because big collisions between players are more similar to getting in a "severe car accident"[1] which is tougher to guard against.
(I am convinced that regular bicycle helmets are crap, as there are no decent standards or crash tests being done with them. The least I would call an improvement is a downhill helmet, a motorcycle helmet would be preferred as speeds for severe crashes are quite similar.)