This is a permanent, not a spell. It's rather silly to make it a spell when this is analogue to every permanent in the game, and not an analogue to any spells.
Kitsune Regneration is a spell, creatures that comes after you cast
Kitsune Regeneration are not affected by Kitsune Regeneration.
I have absolutely no idea what that means.
If you cast kitsune regeneration, then another creature, that creature is not affected by kitsune regeneration.
In that aspect, it differs from a permanent.
But, it's still basically a Permanent, it just doesn't affect new creatures. It doesn't make sense to keep it as a spell; that's like suggesting that Pillars and Towers become spells that last multiple turns and generate quanta. It just makes more sense to have this as a permanent and word it so that it shows clearly that new creatures are not affected.
Alternatively, it could be add a "regeneration" counter to all your creatures in play. That would work as a spell.