First there is the precedent of allowing the previous deals. This is only 2 years longer than the previous longest contract and takes the player only two years older than the precedent. There is no maximum contract length clause in the CBA. Further there are players in the current league around the end age. Recchi will be 43 this year, and Chelios played last year at 47/48. Finally the payouts at the end of the contract are generally in line with what a 40+ year old would make in the league (the minimum).
The Pronger deal isn't an example of cap circumvention because it's a 35+ contract. Even if it wasn't a 35+ contract it's still nowhere near as bad as Hossa or Kovalchuk's deals.
I'm guessing the NHL nixed this because Kovalchuk's deal was far more blatant and severe cap circumvention then any other deal that's happened before. Kovalchuk would also be 44 by the end of the deal, that's a lot more unrealistic then 42 (which is when Hossa's deal ends).
I'm sure that if Lou can figure out how to lower the deal by a couple of years and still maintain a similar deal then the NHL will accept it.
It's really odd that the NHL has a "Kovalchuk a Devil for life" article on nhl.com though.
it artificially lowers the cap hit of a player, that my friend is cap circumvention.