More precisely, a standing policy that researching patents is forbidden is prima facie evidence that your employees couldn't have possibly known about an existing patent. That means that a plaintiff suing for willful infringement will need to find evidence that someone went out of their way to ignore the policy. That might be quite difficult.

(Of course: not a lawyer, this is not legal advice)

Wow, I'm amazed that this would hold up. That's like saying that if I drive with a blindfold on, I couldn't possibly have willfully caused an accident because, as a matter of policy, I couldn't have been aware of the other cars on the road.

A more accurate analogy is a company policy that strictly prohibits driving.