Interview rules in the NFL - why?

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 It just seems like an outdated rule that protects owners that don't have a strong front office structure and lack the ability find replacements.

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 It just seems like an outdated rule that protects owners that don't have a strong front office structure and lack the ability find replacements.

First, I have never read the collective bargaining agreement or the NFL by-Laws so this is pure conjecture. Generally, these coaches are under employment contracts. These contracts carry specific terms and a number of clauses that can greatly limit the employee. For instance, coaches often sign contracts that are for a period of years (i.e. a 3 year contract). The owner is obligated to pay out that contract unless specific issues arise that allow them an "out". SO if the coach is let go after 2 years, they still get paid a third. In return for these $ guarantees, employment contracts can limit who you work for if you leave the job and are still under contract. Look at it as not being able to have it both ways. You can't be guaranteed pay and be "disloyal" for lack of a better term.
Now, it appears that NFL coaching contracts have some sort of out provision that allows a coach to leave for a "better" job. In return for this provision, the teams must be notified and are appraised of the situation so they can possibly make a counter-offer. It may also be some sort of agreement among the teams that if they are properly notified, they will allow the interview and not seek to void the contract.
These terms are actually quite good. In the "real world" of employment contracts, meeting with another employer while under contract could void your current contract all-together. In addition, many employment contracts contain non-compete agreements that bar you from working in the same field as an ex-employer for a certain period of time.
Of course, this is all conjecture, but it seems that this is the system in place.

Remember as well that titles are routinely thrown around in the NFL. Greg Blache just got promoted to Defensive Coordinator from his old job as... Defensive Coordinator. He served under Assistant Head Coach - Defense. That would be a little confusing, compared to the offensive side of the ball, where we had an Assistant Head Coach - Offense, an Associate Head Coach - Offense, an Offensive Coordinator, and several coaches without specific titles, in addition to position coaches. This was, of course, under one of history's great offensive minds.

So, to recap, they don't allow it because the title doesn't always match up to the position.