Up until 2000, export control regulations made it illegal for a U.S company to release cryptographic tools internationally with a key size greater than 40 bits. Lotus Notes got around this by making a deal with the NSA to let them encrypt an additional 24 bits with an NSA key to allow greater security from everyone else but let the NSA still access things easily. At least some have speculated the Windows NSAKEY was for a similar purpose.
Let's be honest, Why would NSA force MS to add a key like that, in such a way?