It makes perfect sense in my eyes. First of all, higher difficulties put the player through more peril and the rage income is lower - the description in the game says so. Second, a quick victory is a difficult challenge. If one wants to complete that challenge, he/she must dedicate a lot of time and energy to that goal with the resources he/she has at hand.
As for the restoring rage/mana to full, that looks way too unbalancing in my eyes. That sounds as if you run a 42km marathon, cross the finish line and suddenly be ready to run another 42km? You are supposed to feel weakened after a battle - although the game itself gives plenty of options to be ready for the next one...
If rage/mana was to be at max after each battle, that would benefit the mage the most. This means you can start the next battle with 2x fire rain/geyser + underground blades and lay waste to most of the enemy troops, leaving half of them burning (or frozen) for even more damage, making the battle easy to win even on impossible. That would be way too unbalancing in my eyes and everyone will play the mage, rendering all other classes useless.
So why not make King's Bounty: Ruled by the Mage and be done with it