I've heard from several sources that the company has no say in the matter. That it's basically rule by fear. That said, thrusting 83b paperwork upon a company that doesn't "allow" it isn't the best way to start your new job.
Also, Peter Thiel always told his portfolio companies to allow 83b elections. It's smart for the employee and the employee. I guess it's a changing of the guard from the Founders Fund types to the YC mania today.