Interview rules in the NFL - why?

[Bill B]

Can someone explain to me why the NFL prohibits teams from interviewing assistant coaches without first being granted permission unless it is for the head coaching job? For instance the Skins had to get permission from the Seashawks to interview Zorn for the offensive coordinator position even though this would have been a promotion from being a qb coach. To me this current rule restricts a coaches ability to move up the ranks and needs to be changed. If you are a position coach and done a great job you should have the opportunity to interview for an offensive/defensive coordinator position. Heck I am not opposed to being granted interviews even if it is a lateral move. It just seems like an outdated rule that protects owners that don't have a strong front office structure and lack the ability find replacements.